LEGO Universe takes place in an alternate universe populated by LEGO minifigures. Many years ago, a team of minifigures went on a great journey to seek the last essence of pure imagination. Having found it on the mysterious planet Crux, the mentally questionable leader of the expedition, Baron Typhonus, used the imagination to accidentally create a giant, chaotic spider to demonstrate the power of imagination. However, the creature backfired on Baron, so took him into the source of pure imagination, creating a dark force known as the Maelstrom.
This being/amalgamation of dark imagination is able to turn otherwise innocent minifigures and creatures into Stromlings.