Firehorse

Darkning (When Firehorse enters the battlefield, put a 1/1 black Imp creature token with flying onto the battlefield. It has "This creature can't block.")
When Firehorse dies, exile it, then put a 2/1 blue Spirit creature token onto the battlefield. It has "Creatures your opponents control get -1/-0."
An imp as it enters this world. A damping spirit as it leaves.