Barzal, the Impetuous

Autonomous (This creature can't attack or block.)
At the beginning of combat on your turn, if Barzal,
the Impetuous is untapped, choose an opponent at
random. Barzal, the Impetuous deals damage equal
to its power to that player unless that player puts a
+1/+1 counter on Barzal and sacrifices a creature.
At the beginning of each opponent's upkeep, flip
a coin. If you lose the flip, that player gains control
of Barzal, the Impetuous and untaps it.