Armageddon

The spell of Armageddon is truly devastating. Once it is cast, it destroys all life on the world, with exception of the spellcaster, leaving only a barren wasteland behind, devoid of any life. Only the biggest fool, full of madness, would dare to cast this spell, as it would be the end of the world as we know it.

You are forewarned!

History
Over 700,000 years ago, the Xorinite Wisp gave it to a mage named Zog. Zog was foolish enough to cast it, and all life was destroyed. The spell re-appeared in Ultima VI, where the Wisps gave it to the Avatar as a gift. In Ultima VII, it could be bought from Rudyom in Cove or found in the spellbook of a random evil mage near the end of the game. The spell also appeared in both Underworlds, with the same effect on the environment.

Trivia

 * It should go without saying that this spell serves no practical purpose. It was put in the games merely to tempt the player into using it, just to see what happens.
 * In theory, it should still be possible to win Ultima VI even after casting Armageddon. The goal of the game is to return the Codex of Ultimate Wisdom to the Void, and this task is not hindered by the destruction of all life in Britannia. However, the programmers anticipated that players hell-bent on "breaking" the game would try to do exactly that. Therefore, sending the Codex into the Void becomes impossible once Armageddon has been cast.  This shouldn't be surprising; after committing global mass murder, failure is the least such an unvirtuous Avatar deserves.
 * In the story of Ultima IX, Armageddon plays a crucial role at the end of the game; the Avatar must cast it to defeat the Guardian, sacrificing his own life in the process. It does not kill every living being in Britannia.
 * In the original plot of Ultima IX, Armageddon destroyed Britannia itself, though its inhabitants were spared thanks to the actions of the Avatar and Lord British.
 * Some people had immunity to the Armageddon spell, notably Lord British and Batlin.
 * In the Ultima Underworld games, Armageddon also destroys all doors and stairways in the maze.
 * In Ultima VII Part Two: Serpent Isle it was replaced by the much less destructive spell, Imbalance.
 * In Ultima VIII it kills the caster as well.