Vyrrax, Baleful Serpent

Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
When Vyrrax, Baleful Serpent enters the battlefield, put a -1/-1 counter on each creature
an opponent controls.
Whenever a -1/-1 counter is put on a creature,
put a +1/+1 counter on Vyrrax.