Duskmantle Agent

Mindwipe (Whenever this creature deals combat
damage to a player, that player puts that many cards
from the top of their library into their graveyard.)
Whenever one or more cards are put into an
opponent's graveyard from anywhere, you may
gain control of target creature with converted
mana cost less than or equal to the amount of
noncreature nonland cards in that player's
graveyard.