Living Spells
Living Spells are a kind of monster which emerged from the Eberron setting of Dungeons & Dragons. Born as a result of the Day of Mourning, Living Spells are... well, the name kind of gives it away, don't it? They're self-perpetuating spells, which exist only to continually cast themselves over and over again. Since most of these are destructive battle spells... well, that basically means killing shit.
4th edition established in Dragon Magazine #419 that there are actually some living spells that have evolved beyond this state and achieved full-fledged sapience, which are now divided into groups as they try to figure out what they actually want to do now that they're alive.
4e also established the existence of Apocalypse Spells, which are basically Living Spells created when god-tier ultra-magic gets kind of hinky. Like, the Rain of Colorless Fire spawning its own unique brand of elementals kind of hinky..