Weapon of X-Slaying

A weapon which gets extra bonuses (possibly including Armor-Piercing Attack or One-Hit Kill) against a specific enemy type, without resorting to Depleted Phlebotinum Shells (e.g. silver vs. werewolves) or Elemental Rock-Paper-Scissors (e.g. using fire against an ice monster).

There may be some overlap with Achilles' Heel; in such cases, consider the weapon's effectiveness against things that aren't the Nigh Invulnerable critter.

Unique-named weapons with this kind of property often come with Exactly What It Says on the Tin naming.

Compare The Hunter, who is a person dedicated to slaying a specific enemy type.

Anime and Manga

 * Record of Lodoss War took this to an extreme, with a set of magical halberds enchanted to slay one specific dragon.
 * One story arc in To Aru Majutsu no Index revolves around a weapon called the Stab Sword, which is able to kill immensely powerful magic users.
 * Technically the Zanpakuto swords from Bleach are used to release dead souls and cut down Hollows.
 * Wargreymon's spiked gauntlets are called "Dramon Destroyers"; they give him a particular advantage against MetalSeadramon and Machinedramon.

Literature

 * The eponymous weapons of the Dragonlance setting were exceptionally effective against dragons.
 * In the Gotrek and Felix novels, Felix's Situational Sword is especially effective against dragons.
 * In Lord Demon, many of the demons have weapons that are attuned to kill them specifically. Kai Wren shows up to one duel wielding the sword made to kill his opponent, and the wounds he sustains don't heal properly. Kai Wren also has an Oh Crap moment when he realizes just who has the weapon made to kill him.
 * The Scourge of Rkard is amazingly effective against the Dragon and the various Sorcerer Kings.

Tabletop Games

 * The magic item lists in Dungeons and Dragons have a lot of these - "sword +2, giant slayer", "arrow of slaying", etc.
 * Third Edition generalized these with the "bane" effect (+2 to hit and +2d6 damage against specified enemy).
 * Inverted in Munchkin by the "sword of slaying everything except squid". It gives a substantial bonus but against squid (there are two in the decks so far - karate squid and Squidzilla) the fight is automatically lost.

Video Games

 * The golem staff in Wizard's Crown, which is just a normal staff unless you attack a golem with it, in which case it ignores armour and has its damage rating tripled.
 * Fire Emblem has a lance called Horseslayer which is highly effective against cavalry. There are also the Axereaver, Swordreaver and Lancereaver weapons, which reverse the weapon triangle by being effective against the weapon referred to in their name.
 * In Radiant Dawn, Micaiah gets the character-exclusive Thani, light magic of Heavily Armored Mook slaying.
 * Final Fantasy I and some of its sequels have various weapons of this kind. They didn't work in the first game, however, due to a bug (which was fixed in the remakes).
 * Angband has a set of Slay X properties (Evil, Orc, Demon, Undead, Troll, Giant, Animal, and Dragon), which can be found in various combinations on ego and artifact weapons.
 * ADOM followed a similar model to Angband; the possible values of X in this case included humanoids (which category included some bosses and most elite mooks).
 * Dungeon Crawl has weapons of orc slaying, which simply give a damage bonus against orcs.
 * Several of the artifact weapons in Nethack fall into this category e.g. Ogresmasher, Werebane (which happens to also invoke Depleted Phlebotinum Shells, since it's a silver saber), and Orcrist. Additionally Vorpal Blade, which can One-Hit Kill anything with a head 5% of the time, does so 100% of the time against jabberwocks.
 * Diablo II gives all blunt weapons this effect against undead monsters, and also has weapons with specific anti-undead or anti-demon enchantments.
 * Weapons with bonuses against the Darkspawn, undead, and/or demons are pretty commonplace in the Dragon Age series, but Dragon Age II also has a greatsword with a large bonus against Qunari. Fittingly,  In the following act, a Qunari gives you a sword specialized against humans (probably as a stealth comeback).
 * The Valkyrie Profile series has a whole range of Slayer weapons--they tend to be Breakable Weapons, making them Too Awesome to Use against anything but bosses. The game only rolls to see if a weapon breaks at the start of your enemy's turn, however, so if you finish the battle in one round, it will never make the check, allowing them to last you through the whole game. As the weapons are uniformly a one-hit kill, the only real danger of losing them comes when you're facing more than one such enemy, which happens rarely, and only after you've had the opportunity to pick up multiple weapons of the type.
 * In addition to the Slayer weapons, there are also weapons that have a Death effect based on a given element. With these, the weapon will instantly kill anything that has that elemental weakness, regardless of total HP or defenses. This includes the penultimate boss on the way to the Golden Ending, who can be killed in one round by a sword you picked up around the midpoint of the game.
 * Covenant of the Plume doesn't have the One-Hit Kill weapons, but it still has weapons with bonuses against specific monster types.
 * Age of Wonders: Shadow Magic has a "Dragon Slaying" unit ability, given to Knight and available on weapons for Hero Unit.
 * Runescape has several examples, including Silverlight (and its upgraded form, Darklight) for slaying demons; Balmung, the Battleaxe of Dagannoth Slaying; the Ivandis Flail, used for killing Vyrewatch vampyres; and the brackish blade and brine sabre, which are super-effective against crabs for some reason.
 * Shining Force has a variety of specialized weapons that are strong against specific types of enemies -- the Mermanbuster is strong against sea creatures, the Jagged Flash is strong against flying enemies, the Elf Slayer is good against robots, and so on.
 * Castlevania: Symphony of the Night has the Werebane, which is supposed to do more damage against werebeasts. It doesn't. Apparently they didn't program it correctly...
 * The Legend of Dragoon has two ancient weapons forged to defeat Dragons and Dragoons: one of them was the Dragon Buster sword, capable of piercing the Dragoon armor. Unlike most video game examples, it's your enemy that wields it, handicapping you during your battles against him.
 * In Legend of Mana, weapon upgrades can bestow or remove this effect.
 * EverQuest has a "Bane" effect which increases damage towards a particular species of monsters.
 * Wizardry has weapons dealing double damage to specific types. 7-th has weapons against Dragon, Robot, Undead and, due to a silly bug, Fighter; in 8-th against Android, Beast, Demon, Dragon, Plant and Undead.
 * Crossfire adventure game has slaying weapons, and when a deity blesses someone's weapon, it usually acquires the slaying property vs. the kind of creatures this deity doesn't like.
 * Most games in the Dragon Quest franchise have an assortment of these, doing higher damage against the designated enemy (e.g. weapons with "metal" at the start of the name usually do extra damage when they hit a Metal Slime, while the dragonsbane does extra damage to dragons).
 * In Drakensang, dwarves have the Wyrmslayer Ax (a small hatchet with a very big blade) and also the Dragonslayer Axe, the Dragon-Crushing hammer, the Dragon-Piercer crossbow and the Dragon Spear (as in a trident made for skewering dragons). Yes, before you ask, dwarves hate dragons, and even have something like seven different words to define the death of a dragon.
 * Kingdom Hearts: Chain of Memories played this straight and averted it, by giving various cards elements, using fire type against water-type gives an extra attack boost. However using the same element heals them.
 * Wuuthrad, the legendary battleaxe of the Nordic king Ysgramor, deals more damage to elf races. This applies even if it's being wielded by an elf.

Web Original

 * Mr. Welch has occasionally been reprimanded for forging weapons of party member slaying.