Xoz, Renegade Priest

Infect (This creature deals damage to creatures in
the form of -1/-1 counters and to players in the form
of poison counters.)
Whenever a creature enters the battlefield under
an opponent's control, that player loses 1 life.
Whenever an opponent loses life, put a +1/+1
counter on Xoz, Renegade Priest, then you gain
1 life. This ability triggers only once each turn.