Feyx, Apex of Crystals

Mutate {4}{r/w}{u}{u} (If you cast this spell for its mutate
cost, put it over or under target non-Human creature
you own. They mutate into the creature on top plus all abilities from under it.)
Flying, hexproof
Whenever this creature mutates, artifacts you
cast cost {x} less to cast until end of turn, where
X is its toughness.