Blood Contractor

Autonomous -- Sacrifice a creature (This creature
can't attack or block. At the beginning of each
opponent's upkeep, that player may sacrifice a creature.
If they do, they gain control of Blood Contractor and
untap it.)
At the beginning of combat on your turn, if Blood
Contractor is untapped, choose an opponent.
Blood Contractor deals damage equal to its power
to that player unless they put a +1/+1 counter on
Blood Contractor and sacrifice a creature.