Kirulan Guardian

Islandwalk, swampwalk, trample
Ambush- When a creature with power equal to or less than this creatures power enters the battlefield under an opponents control, you gain 5 life and that creatures controller is dealt 5 damage. Damage dealt in this way can't be prevented or reduced.
{u}{b}: The next creature that enters the battlefield under target players control gets -1/-1 until end of turn.