Muerta, Death's Judge

When Muerta, Death's Judge enters the
battlefield or phase in, it deals damage to target creature equal to its power. If an opponents
creature is put to the graveyard, put a +1/+1
counter on Muerta.
When Muerta becomes the target of a spell or ability, you may pay {x}. If you do, Muerta phases out unless the controller of the spell or ability pays {x}.