Clockwork Chewer

At the beginning of your end step, put a +1/+1 counter on Clockwork Chewer, then mill a card
for each +1/+1 counter on it.
{3}: Until end of turn, Clockwork Chewer
becomes a 0/0 Construct artifact creature with
skulk and "Whenever this creature deals damage
to a player, that player gains control of it."
(It can't be blocked by creatures with greater power.)