Master of Shadows

Whenever a creature card is put into an
opponent's graveyard from their library, you may
pay {1}. If you do, create a 1/1 blue and black nightmare rogue token.
If a creature you control is either a nightmare or
a rogue or both, they get +1/+1. If a creature you control is either a nightmare or a rogue or both
and an opponent has at least 10 cards in their graveyard, they get +2/+2.