Mercury, Queen of the Seas

Pactbound (When you cast this spell, you may exile
a Pactholder creature you control. If you do, reduce
this card's mana cost by {x}, where {x} is the exiled
creature's converted mana cost. When this creature
dies, return the exiled creature to play.)
Islandwalk, Hexproof
—
"I am Mercury, mistress of the tides and ruler of
the waves. I shall be the ocean that carries you!"