Unholy Nuke

Many works have the Holy Hand Grenade: a purifying, holy blast of light and goodness which eradicates evil in sight, turning it into a dusty pile of ash.

However, what happens if it gets reversed?

The Unholy Nuke tends to be a power fit for a Big Bad or Complete Monster: a devastating magic/technique/ritual fueled by unholy power, often the Evil Counterpart to a technique used by the good guys. Usually it involves Black Magic and darkness, but other elements can play a role as well. Bonus points if it's Powered by a Forsaken Child. It may be a Dangerous Forbidden Technique. Sometimes an Evil Knockoff will use this as a dark mirror of one of the hero's own powers. In Video Games, expect to see it used by the Final Boss.

See also Light Is Not Good, Casting a Shadow, Person of Mass Destruction. May or not be part of a Yin-Yang Bomb. Contrast Holy Hand Grenade.

Anime and Manga

 * Ryoga's Shishi Hokodan from Ranma ½, which is fueled by negative feelings and highly destructive.
 * In Naruto, the attacks from Naruto's 4 and 6 tailed forms may count as this.
 * In Fairy Tail Jellal's magic Altearis consist in an extremely devastating orb. In the anime it sucks all the shadows in the room towards it and makes Natsu sick just by standing near it. Later, Brain 's spell "Genesis Zero" is pretty much this, only with a tsunami of black shadows.
 * Hades's Grimiore Law. Later, in the same saga,.
 * Death Magic.
 * Yaiba has the Devil King Sword's Full Moon Blast, used by Onimaru. Furthermore, all the attacks performed with the Devil King Sword are fueled by the negative feelings of the wielder.
 * Lina Inverse's Dragon Slave spell in The Slayers (when it is not Played for Laughs). Just listen to the incantation.
 * In Dragon Ball GT there is the Revenge Death ball, powered by the hate of Baby's followers. Also Omega Shenron's negative energy ball, made from all the bad karma left over from all the sins on Earth.
 * In One Piece there's God Eneru with his infamous, island-annihilating Raigou.
 * In Bleach Barraggan's Respira, Ulquiorra's Cero Oscuras and Lanza del Lampargo, and Hollow Ichigo's horned Cero.

Fan Fiction

 * An Italian Dragonball Z Fan Fiction features Super C17 performing the "Lurkodama", described as the "Hell version" of the "Genkidama" and based on negative energy. Is used to form a Yin-Yang Bomb along with Gogeta's Genkidama.

Films

 * In Willow the main villain Bavmorda wants to use a ritual whose end is to wipe the victim away from existence.

Literature

 * The Darkhallow ritual from The Dresden Files is the ultimate necromantic spell that kills everything, both living and spirit, within several miles' radius, then turns the caster into a Physical God.
 * There's also the bloodline curse from Changes, which kills everyone related to a sacrificial victim.
 * The Thought Bomb in Darth Bane Path of Destruction is one such example; using Sith energy, it obliterates all Force-sensitives (and for those close enough, all life-forms) within its radius,
 * Balefire from The Wheel of Time. It doesn't kill its victim -- it burns away their very existence from reality, retroactively.  In at least one case, the other combatants can no longer remember why certain things, caused by the targets of Balefire, happened.

Tabletop Games

 * In Deadlands: Hell on Earth, these are actual demonic Magitek nukes. That destroy the world.
 * Dungeons & Dragons
 * The Talisman of Ultimate Evil. In the hands of an Evil High Priest it could be used to open a flaming crack at the feet of a Good priest and send him or her to the center of the planet.
 * Unholy Water causes damage to creatures from the Upper Outer planes (the planes of Lawful Good, Neutral Good and Chaotic Good).
 *  Warhammer 40,000 is full of these. Basically any time Chaos is involved, expect Unholy Nukes to be flying thick and fast.
 * Exalted has a few of these to contrast the more enlightened ones:
 * The Solar Circle Sorcery spell Total Annihilation isn't automatically unholy, given who wields it... but it's still the equivalent of a fantasy nuke, and relies on calling on a portion of the energy of Ligier, Hell's green sun.
 * More directly descended from Ligier are Malfeas's more...extreme Green Sun Nimbus Flare expansions, and Demon Emperor Shintai. Both of which contain actual radiation sickness on hell steroids.
 * Mouth of the Void, a Void Circle Necromancy spell that does hideous amounts of damage to all creatures over a large area. Given its level, it can only be learned by Abyssals, who usually aren't on the side of goodness and light. Usually.

Video Games

 * The spell "Flare" from Final Fantasy I can be this, as its counterpart is the "Holy" spell from the White Mage. Also in Final Fantasy II Flare takes the form of a dark nuclear blast and some powerful evil enemies can cast it.
 * From Final Fantasy VII there's the Meteor spell cast by Sephiroth. Note that in previous games Meteor was far more neutral.
 * The "Dark Holy" spell from Final Fantasy Tactics, used exclusively by the highest order of demons, fits this bill to a T.
 * In Mass Effect, when possessing a puppet, the Reaper Harbinger has a special version of of the "Warp" ability that probably qualifies
 * In Jedi Knight Jerec can use the "Destruction" Force Power. It's a Dark Side attack that consist in a fiery ball of flames and negative energy that destroy everything on its path.
 * In Heroes of Might and Magic, Death Ripple damages all non-undead units on the battlefield. Unholy Word from V does the same, but also excludes demons.
 * Spellforce: The Black Magic sub-branch of Death has the quickest damage output in the game.
 * Secret of Mana had the unholy nukes dark force and evil gate.
 * In Suikoden, pretty much all of the Soul Eater's spells are considered this.
 * The Ultimate Insult from Escape from Monkey Island is one of the most powerful Voodoo artifacts around, though is not that lethal for the body, but annihilates the spirit.
 * The Demonic Megiddo from the Castlevania series is the Dark Lord's most powerful attack, and tends to be a One-Hit Kill super powerful spell used mainly by Dracula or Death. In Curse of Darkness, one type of magician Innocent Devil can cast a similar spell dubbed Demonic Disaster.
 * The Carronnade from Breath of Fire IV is possibly the most horrific example of this trope ever, fueling its Unholy Nukes with soul-crushing torment and turning that anguish into a weapon of mass destruction that turns the area for miles around into a nightmare that gives Hell a run for its money.

Web Comics

 * In Eight Bit Theater Black Mage mentions that he gained his Hadoken Attack (essentially an all-purpose Nuke) through sacrificing children to his wanton gods of evil. This surprises no one.
 * It's also mentioned that it's powered by consuming love from the universe, and that the divorce rate rises slightly every time he uses it.
 * Dominic Deegan has a rare example of one used by a good character. Dark Soul Burst is a spell powered by anger and hatred, and is typically used by Luna when someone REALLY pisses her off. The first time it's used, it knocks out Siegfried, a rather strong royal knight, in a single shot.