Vaal Hazak, Tainted Fiend

Flying
{b}{g}: Place an effluvium counter of target
creature you control.
Creatures with effluvium counters have wither
and "Whenever this creature deals damage to a
player, that player loses half his or her life,
rounded down."
"Vaal Hazak serves its purpose in the life cycle of the
New World, but now that it has stirred from its nest of decay, it threatens to ruin the whole ecosystem."
--Cheif Ecologist