Viceroy of Bolas

Prowess (whenever you cast a noncreature spell, this
creature gets a +1/+1 until end of turn.)
{x}, Sacrifice a creature: create a 0/0 black army
creature token. its power and toughness is equal
to the mana spent to activate this ability. you may
remove a +1/+1 counter from Viceroy of Bolas
rather than paying for this abilities mana cost
"an efficient way to harvest essence."