Hexerei
Eine Kreatur deiner Wahl erhält +3/+3 bis zum Ende des Zuges und muss in diesem Zug geblockt werden, falls möglich.
If each creature the defending player controls can't block for any reason (such as being tapped), then the affected creature isn't blocked. If there's a cost associated with blocking the affected creature, the defending player isn't forced to pay that cost, so it doesn't have to be blocked in that case either.