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.
3rd edition has rules for creating custom living spells by applying a template to a spell or multiple spells in the same way that you would modify a monster by adding a template. These living spells are always classified as an Oozes.
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.
A single living spell appears in 5th edition book, Dungeon of the Mad Mage. The three more show up in Eberron: Rising from the Last War. They are now classified as constructs instead of oozes..