Agent of the Shadows

Enlist (As this creature attacks, you may tap a
nonattacking creature you control without summoning
sickness. When you do, add its power to this creature's
until end of turn.)
Agent of the Shadows enters the battlefield with
a hexproof counter.
Whenever Agent of the Shadows enlists a
creature with power 4 or greater, remove a
hexproof counter from it.
Agent of the Shadows cannot be blocked.