The Blob

Shroud, indestructible, wither, trample

When The Blob attacks, defending player sacrifices a creature; if they do, put a +1/+1 counter on The Blob.
Whenever a creature an opponent controls dies, if it had a -1/-1 counter on it, put a +1/+1 counter on The Blob.

It creeps, and leaps, and glides and slides across the floor.