Morph Weapon

A Rule of Cool weapon. Generally magical, but can be explained with nano-science instead. Basically, it shapeshifts into whatever form is most suited for the situation. It might be magically guided by telepathy, or even have a mind of its own! Whatever the case, it tends to be significantly more Badass than any boring old stays-the-same-all-the-time weapon.

A type (and thus subtrope) of Swiss Army Weapon. Compare Shapeshifter Weapon, where the character's own body is a form-changing implement of destruction, and Retractable Weapon. And of course, this weapon goes very well along with Adaptive Armor.

Anime & Manga

 * Neon Genesis Evangelion: The Lance of Longinus.
 * The zanpakuto in Bleach.
 * The Kakugane in Busou Renkin.
 * In Panty and Stocking With Garterbelt, Panty and Stocking's namesakes work as these.
 * Every device in Magical Girl Lyrical Nanoha has at least three forms, sometimes changing its weapon class. As a bonus, one of these forms is always its user's own Transformation Trinket, so there's no need to invest in more items.
 * Musica in Rave Master wears around his neck a silver skull which, due to his particular brand of abilities, he can manipulate and change into any form he wishes, most commonly a spear.
 * The protagonist, Haru Glory, also possesses a sword that has ten different forms, including a simple iron blade, a weapon that explodes on contact, or one that can cut through smoke, magic and other non-cuttables. Two Big Bads possess Evil Twin versions of Haru's sword.
 * Also in Hiro Mashima's next work, Fairy Tail, Knightwalker wields the Ten Commandments in spear form.
 * Rosario to Vampire features Moka's younger sister, Kokoa. She can command the Combat Commentator bat to turn into a mace, a mallet, and... well, a bat.
 * Vash and Knives' handguns turning into the insanely powerful Angel Arms in Trigun.
 * The Aqua Ribbon in Yes Precure 5, which only really takes the form of a ribbon in the merchandise.
 * As the narrator explains, the Carrot Weapon that Mina uses in Getsumen to Heiki Mina responds to her thoughts. As such, it can be whatever the heck kind of weapon it wants to be -- and it has been quite a lot.
 * Black Star's weapon Tsubaki in Soul Eater has multiple forms: really big shuriken, kusari-gama, smoke-bomb, ninja blade, and, later, . She, and the other Weapons, also counts as Equippable Allies and Shapeshifter weapon.
 * Ed's arm in Fullmetal Alchemist is effectively one of these, not due to any facet of its construction, but because Ed constantly uses his alchemy to change it into various weapons, most commonly a simple blade, though he has been seen changing it into more advanced forms, such as a BFG, or even altering its chemical composition to make it more effective against certain enemies.
 * Mahou Sensei Negima
 * Jack Rakan has
 * In Yu Yu Hakusho, we have the Toguro Brothers. The Elder Toguro can shapeshift into any weapon the younger, muscular Toguro wants to wield; sword, shield, you name it. When the Elder fights alone, he's a Shapeshifter Weapon.
 * The "Ultimate Weapon" is the main MacGuffin during an early story arc of Space Adventure Cobra. When finally discovered, it looks rather inconspicuous: an ovoid fitting in the palm... from which an eye open. But then, if this eye catches sight of any weapon, it can turn into a copy of it... whether a sword, a gun or a tank. And the Weapon can also turn its wielder into a freaking giant along with its own scaling up. When confronted with this, Cobra takes great care of avoiding that the Weapon could become a spaceship...
 * Subverted in Hunter X Hunter, where 's weapon Crazy Slot, upon activation, turns into one of nine very different forms... at random.
 * Flame of Recca: Kaoru's madōgu is a "puzzle weapon" and can be assembled five different ways; given how the weapon looks after the changes, how exactly this happens is unknown, as one form is basically a lance and another involve a long chain.
 * Flame of Recca: Kaoru's madōgu is a "puzzle weapon" and can be assembled five different ways; given how the weapon looks after the changes, how exactly this happens is unknown, as one form is basically a lance and another involve a long chain.

Comics -- Books

 * Defunct comic Operation: Strikeback!'s main character Rascal had a red and white striped scarf that morphed into whatever Rascal wanted or needed at any given moment. From a frying pan to a chainsaw. Yes, a chainsaw.
 * The Tactigon in Avengers: The Initiative
 * In Annihilation Conquest, Wraith's polymorphic weapon took on a variety of forms, including a gun, a whip, and a small blade, and could somehow deactivate/disarm other weaponry.
 * The artifacts from the Top Cow universe (The Witchblade, The Darkness, and The Angelus) act as both morphable armor and weapons.
 * Death's Head II's right-arm implant.
 * DC Comics: During the Druspa Tau story arc of Hawk & Dove (1990), tridic metal that let the wielder transform it into any desired weapon, using either Name Magic or a laboriously detailed knowledge of the weapon.

Fan Works

 * In Kyon Big Damn Hero one of the slider's enemies is equipped with two of those. Later, Kyon and Fujiwara take hold of them.

Films -- Live-Action

 * The Golden Army: Prince Nuada has a spear that can change into a dagger and vice versa.

Literature

 * The ur-example may be the eponymous object from Larry Niven's short story "The Soft Weapon", which was later shoehorned into the Star Trek 'verse for an episode of Star Trek the Animated Series.
 * Iridea's Tear in The Threat from the Sea. But it's an artifact so ancient that even legends about it are mostly forgotten, a Clingy MacGuffin, it does not accepts just everyone and requires proper rite for complete binding to its new owner.
 * The Great Weapon Godslayer in Dragaera. However, while she's sentient, there doesn't seem to be much logic behind the forms she uses, much to the annoyance of her wielder.
 * One of the Sector General novels has a living continent whose only manipulative tools are silvery symbiotic organisms of organic metal, which respond to thought to reshape themselves. At close range, other sapient beings can override the vast but diffuse mental control of the landcreature.
 * The Platinum Flute in the Apprentice Adept series can become any weapon or musical instrument.
 * The Sartan dagger in The Death Gate Cycle not only turns itself into a weapon powerful enough to face whatever its wielder's enemies are carrying, it also changes the wielder himself if he can't handle the dagger's new form.
 * For example: an elf wielding the dagger had to face another elf with a broadsword. The dagger changed into an even bigger broadsword, which was so heavy the elf couldn't hold it -- so the dagger gave the elf two new arms. (This is when the elf died of shock, by the way.)
 * Sometimes, if the wielder is facing an opponent more powerful than the dagger, it will actually summon something capable of defeating the opponent, rather than turning into it. As a result, a protagonist finding himself threatened by a bunch of Knight Templar demigods accidentally used it to summon a swarm of omnicidal undead, which didn't improve the situation any....
 * Dom Sabalos of The Dark Side of the Sun has a "memory sword", which is essentially a hilt containing a device which can produce different types of blades, laser and molecular stripper energy weapons, or even a projectile gun that freezes bullets out of atmospheric moisture.
 * The Mesmerist shiftblades in The Deepgate Codex are souls of the dead conditioned to serve as morph weapons.
 * The Dark Angel trilogy has Curoch, which can be made to change into whatever form best suits it.

Live-Action TV

 * Kamen Rider Kuuga's alternate forms can change random objects into their Weapons Of Choice, with the caveat that they have to resemble the weapon in question; Dragon Form's staff and Titan Form's sword can come from any stick-like object, but Pegasus Form's bowgun requires some form of gun.
 * Kamen Rider Den O's DenGasher is made up of four separate parts which can be combined in various ways to become a myriad of weapons including a sword, spear, ax, gun, boomerang and hand-ax, and naginata. Movie Rider G Den-O's DenGasher can become a handgun and a jutte.
 * Definitely goes with Adaptive Armor; Den-O's forms are made from the same armor pieces rearranging, just as the weapon rearranges.
 * Evil Twin Nega Den-O has the same changing weapon as the regular Den-O, but no form-changing.

Tabletop Games

 * Dungeons and Dragons
 * Morphing is a magical ability that can be put on a weapon, allowing you to change the weapon within its size category (Light, One-Handed, or Two-Handed). However, since attack range and actual weapon properties stop becoming issues by the time you can have a +3 sword (+1 enhancement, +2 morphing), it is one of, if not the most, useless abilities in the game.
 * Also available is the Rod of Surprises, which can change into one of several weapons, each being close to the concept of "decent-sized stick with a blade on one end". It has a few other functions, but those are the main ones. A DM may also allow a player to apply the Morphing enchantment as well. Since the Rod can change into at least one weapon from all three categories, the rod can now become, with a little prep time, any melee weapon ever published. Of course, all the good ones require special training, but that can be solved by taking a level in Warblade, which actually has Instant Expert as a class feature.
 * Which was, of course, predated by the Rod of Lordly Might, introduced in 1E, where buttons on the rod allowed the wielder to turn it into a mace, sword, battle axe, or a spear of length between 6-15 feet. Then, there was the Rod of Lordly Might or Might Not from Dragon magazine which included the weapon form of +3 backscratcher.
 * There's also the Soulknife Class, a psychic character who weilds blades of psionic energy. They begin with a short sword and later can throw it, and around 5th level they can start changing what the blade looks like, from a short sword, to a greatsword, to a bastard sword, to two short swords. With feats they can turn their mindblade into other weapons as well.
 * You can also get the Sizing quality, which allows you to change the weapon's size category. Between this and Morphing, a single weapon effectively becomes every weapon ever.
 * Another D&D reference, karach is an element found in Limbo which can be shaped by the strong-willed, and is used by the githzerai, who inhabit the plane (see the Planescape Torment entry below).
 * The Lunar Exalted of Exalted have Moonsilver, which changes and flows constantly. One of the higher level abilities usable with moonsilver weapons is the ability to have a piece break off and burrow to the target's brain. There are also Charms that allow a Lunar to reshape a moonsilver weapon into any other moonsilver weapon they need, be it an axe, sword, or bow.

Toys

 * From Bionicle: the Toa Nuva's new Adaptive Armor and corresponding weapons automatically shape-shift to fit the environment and/or the foes they are facing. Upon teleporting into Karda Nui and finding themselves in midair, the Toa saw that their armor had already grown wings and jets; while the weapons became light-shooting "Skyblasters", perfect for battling the shadowy, bat-like Makuta that had invaded the area. It's worth noting that this is a Merchandise-Driven example; by giving the Toa shapeshifting armor, LEGO would've theoretically been able to create new figures without worrying about them looking different... if BIONICLE hadn't been canned two years later and that wave had pretty much been the last appearances of the characters.

Video Games

 * Devil May Cry has a number of these. In the first one, the Sparda worked like this, changing from a sword, to a spear, to a sickle, depending on your moves. In the third game, the electric guitar would also morph into a sickle when swung. And in the fourth, of course, is the most ridiculous example of them all: Pandora's Box, which is a rare projectile weapon version of this - turning into a bowgun, missile-launcher, minigun, laser, buzzsaw, or even a flying missile platform!
 * In fact, the description of Pandora's Box claims it can take on 666 different forms, although significantly fewer are actually usable in gameplay...
 * Of course, being Pandora's box the last form, 666, is Dante simply letting it drop open. If the chaos gauge is filled up this does absolutely insane damage, for logical reasons.
 * In the doujin fighter game Eternal Fighter Zero, Sayuri Kurata's Magical Girl wand is capable of transforming into a sword, a gun, a mace, and even a piledriver.
 * Project Snowblind has a variety of weapons, all of which have secondary fire modes that allow them to fill a role that is completely different from their primary role. Examples include a sniper rifle that doubles as a Mind Control gun, the silenced pistol that can fire smart missiles, and the ricocheting flechette gun, which could unleash a swarm of taser drones. One might say that instead of morph weapons, Project Snowblind has morph ammo.
 * Neverwinter Nights
 * Dream Dagger in Neverwinter Nights 2: Mask of the Betrayer.
 * Enserric the Longsword from Neverwinter Nights: Hordes of the Underdark is a limited version. The weapon can morph itself into few other forms, but after pulling it from its former wielder's fingers, it's stuck in that form.
 * Planescape Torment
 * Dak'kon's karach blade is shaped by his thoughts; based on your conversations with him, it changes form.
 * And there's the Entropic Blade, which can be changed into whatever weapon the PC is most skilled with.
 * Soul Series
 * Necrid in Soul Calibur II has a glowing ball of Pure Energy that can become a daisho, zweihander, or battleaxe. He can also do a whip sword or simply expand the ball and smack you with it.
 * The Soul Calibur and the Soul Edge can also change form to fit their wielder.
 * World of Warcraft
 * Certain Epic quality weapons (before the expansion pack) used to be Morph Weapons (Benediction-Anathema is a good example). This mechanic is no longer used, however, due to problems with enchanting such items.
 * Another system was introduced instead, the heirloom items. These items can be passed between any character on the same account and server and scale with the characters level. Some of them even change armor type to cover the fact that several classes get a new armor type to use at level 40 (leather->mail and mail->plate).
 * There's also (with the second expansion) Quel'Delar. Okay, it's always a sword, but in game terms what's far more important is that when it is finally gained after a long quest chain, it "adapts to its wielder" and you can choose what stats it has, which matters more than the actual physical shape it takes. (If you can't use a sword at all, you'll get a different weapon of equal power that I guess they just had lying around.)
 * The Komato GEN2 Pre-Loader uses nanotech to assemble its ammo on the fly, allowing its wielder to open fire with anything -- from machineguns to death beams to the MPFB Devastator -- with absolutely no warning beforehand. The same can be said of the Tasen replica wielded by Iji herself.
 * Kingdom Hearts
 * Keyblades do this in two different ways - first, swapping keychains around gives it a new "look" based on a certain world or character, with different bonuses in terms of strength / reach / magic. When the Birth by Sleep characters showed up, things started getting crazier.
 * Lingering Sentiment's Keyblade changes from a keyblade, to a whip, to a bow and arrow, to a giant cannon, to a HOVERCYCLE!, by contrast, can only use the cannon and cycle modes as a playable character.
 * Keyblades in general can do this although only the Birth By Sleep characters know how to make use of it so far, and of them only Terra and Aqua use it for anything other than a vehicle mode - Aqua has her own bow-and-arrow mode, but can only use it in Deep Space and Mirage Arena games. Each Keyblade seems to have their own specific modes (Terra's becomes a bike, while Ven's becomes a Hoverboard) based on their original 'base' mode.
 * Now in the Final Mix version of Birth by Sleep, the new bonus boss can change his Keyblade into shin guards, a large claw, an arm cannon, a floating shield and even a large pair of wings.
 * Jak and Daxter: Jak's Morph Gun. Really, the name says it all.
 * Zero's Z-Saber in Mega Man Zero can also be a spear, grappling hook, tonfa, magazine for a pistol, or shield you can throw.
 * In Mega Man ZX and its sequel however, the ZX biometal (which is based primarily on the Mega Man Zero era Zero) has a single weapon: the handle of the Z-Saber can tilt 75 degrees, turning it into a chargeable buster pistol (this is only really apparent in cut-scenes, however).
 * While the weapon itself doesn't change, Mega Man's Arm Cannon can fire a variety of different weapons thanks to the weaponization of his Variable Tools System. In the sequel series, Mega Man X, X's X-Buster does much the same, but, when he gets the inevitable X-Buster upgrade, he can also charge the weapons to get entirely different effects (such as Chameleon Sting, which is stated to fire powerful optic lasers, making him functionally invincible when it's charged up).
 * Mana Khemia Alchemists of Al Revis
 * The main character, Vayne, has his cat transform into a sword and armor. And then it further transforms into an even larger sword, among other things...
 * And in the sequel, Mana Khemia 2 Fall of Alchemy, both of main characters transform their weapons during their Intimate Strikes. Raze transforms his sword of light into a spear, and Ulrika transforms her morning star into a spiked hammer.
 * Jude's ARM in Wild Arms 4, "Shapeshifter", typically takes the form of a one-handed pistol, but can also transform into a sword or a two-handed beam cannon, among other things.
 * If what Peter Molyneux says is of any indication, weapons in Fable III will alter their shape depending on what you slay with it.
 * Your Silent Protagonist in Gods Eater Burst wields a weapon which shifts between a BFS and BFG.
 * From the Guilty Gear series there's Millia Rage and her hair. Technically, her hair is alive (long story) and she can use it as a weapon by changing its shape and density at will. Millia can turn it into a blade or shield herself with it, and can shoot a projectile from it, among many other things. Her ultimate kill move has her hair locks impale the opponent. Twice.
 * Mog from Final Fantasy XIII 2 can change between sword, bow, and his moogle form.

Web Animation

 * In the Feast Master story arc of Banana Nana Ninja, the Omni-Functional Kitchen Gadget can turn into a variety of cooking utensils and appliances, as well as a Humongous Mecha used in the final battle.

Web Comics

 * In Darken, there's Shard, a sentient, evil sword, who can change himself into whatever form of weapon is most advantageous. Although he's more likely to change into whatever's funny...
 * Dies-Horribly's metal arm in Goblins. It's also an Empathic Weapon with Power Incontinence for a triple-threat.
 * Secret's weapon in Keychain of Creation is one of these, as well as an Empathic Weapon with a taste for blood -- including the wielder's. Yikes.
 * The title character's sceptre in Gorgeous Princess Creamy Beamy. It's supposed to be only able to morph into various eating utensils, but she managed to get around this with her warped logic; "A sawblade could be an eating utensil!"
 * Beisaru's staff, in Juathuur, can get longer. And sharper.
 * Fiona's weapon in Supernormal Step manifests a variety of shapes out of its apparently-fixed handle. So far she doesn't seem to have much control over its shifting.
 * In Axe Cop, Fire Slicer has a sword that can turn into a torch and then back into a sword, so he can set his opponents on fire while he's cutting them in half.
 * Wildcard, the sword-of-a-million-tricks from Tales of the Questor, made this trick number 1,000,001, debuting this new ability by
 * Homestuck: Kanaya and Jane both wield weapons which are capable of morphing: Kanaya wields a lipstick which becomes a chainsaw, and Jane wields a cutting-edge Betty Crocker spoon which becomes a fork with the flick of a button.
 * Kanaya's in particular is a callback to Problem Sleuth, where object/weapon duality was a running gag.
 * Darius in The Cyantian Chronicles got a somewhat high tech "stick" for his 15th birthday.

Web Original

 * The HaLOs in Avatar Adventures. They make Christianity awesome.
 * In Chaos Fighters-Route of Peaks, Cogra's pen has a laser blade and it can change into a gun pod and a shield.

Western Animation

 * The Lotus Blade in Kim Possible.
 * Yang's bamboo sword in Yin Yang Yo.
 * Featured in one of Oh Yeah! Cartoons more cinematic shorts.
 * Xiaolin Showdown does one of these with Omi's Shimo Staff.
 * There is a transforming sword in the old Teenage Mutant Ninja Turtles cartoon. Splinter is the only ninja skilled enough to wield it; otherwise, it sprouts extra blades and such uncontrollably.
 * She Ra Princess of Power
 * She-Ra's power sword can change shape when she says the phrase "Sword to ".
 * Her friend Bow's... bow can change into a lute whenever he wants to sing a song. This doesn't come up very often, thankfully.
 * And let's not forget Hordak, who can shapeshift his own body into any kind of weapon needed.
 * Inspector Gadget's hat could function as one when it deigns working right.
 * The Star Trek the Animated Series episode "The Slaver Weapon" involves Spock, Sulu and Uhura held captive by a crew of Kzinti (did I mention that the episode was written by Larry Niven? Seriously.) while carrying a case of artifacts from a long-lost race called the Slavers. One of the artifacts turns out to be a hand-held weapon that can change form with a twist of a dial, from an energy pistol similar to a phaser, to a telescope, to a vocalizing computer, to a long-range weapon that carries the impact of a small nuclear bomb. Fortunately for the rest of the galaxy, it also have a self-destruct (which the vocalizing computer tricked the Kzinti into triggering). Sulu hypothesized that the device was used for espionage instead of combat, since the average grunt wouldn't need all those settings.
 * Guardian Keytools from Re Boot (Also a Green Lantern Ring), such as Bob's Glitch.
 * In Barbie And The Nutcracker, the Mouse King's sword could transform into any other weapon he wanted, including a crossbow that somehow starts out with ammunition.
 * Galtar and The Golden Lance: the titular weapon is somewhere between this trope and Swiss Army Weapon as it starts off as a stick that looks like a pair of collapsed pummels, extends into a double-sword/lance, can bifuburcate into a pair of swords, or recombine into a more powerful sword.