Nightmare Soul-Stealer

Bestow {1}{u/b} (If you cast this card for its bestow
cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached to a creature.)
Enchanted creature has "At the beginning of
your upkeep, put a -1/-1 counter on this creature
and put a +1/+1 counter on all Shadows and Nightmares on the battlefield."