Proteus Dynamo

Sacrifice Proteus Dynamo: Add {b}.
When Proteus Dynamo goes to the graveyard
from the battlefield, you may pay {x}{b} to return target creature card with a converted mana cost
X from your graveyard to the battlefield with
Haste and "Exile this creature at the end of your
next turn." If you do, the next black instant or
sorcery you cast this turn costs {x}{b} less to cast.
You may use this ability only once a turn.