Shadow Stalker

Adaptability
Shadow Stalker gets +1/+1 for each creature in
your opponents' graveyards. If there are 15 or
more, Shadow Stalker has shadow.
Whenever Shadow Stalker attacks and is
unblocked, untap it and put X +1/+1 counters on
it at the end of the combat step, where X is the
amount of combat damage creatures you control
dealt this combat.