Blue Horror

Blue Horrors are the unholy product of Tzeentch's Pink Horrors. When a Pink Horror is slain, there’s a 20% chance something strange occurs, in that the Horror splits into two identical Blue Horrors, retaining any traits and characteristics of the Pink Horror they spawned from. Alternatively, any given Horror might explode in a cloud of chromatic energy, causing anything nearby to mutate from the raw magic of Chaos.

Source

 * Warhammer Fantasy RPG 2nd ED -- Tome of Corruption (pg. 233).