Changeling Berserker

Changeling (This card is every creature type.)
Haste
Champion a creature (When this enters, sacrifice it unless you exile another creature you control. When this leaves the battlefield, that card returns to the battlefield.)

The List

Lorwyn

Changeling (This card is every creature type at all times.)
Haste
Champion a creature (When this comes into play, sacrifice it unless you remove another creature you control from the game. When this leaves play, that card returns to play.)