Dagon

Whenever an Instant, Sorcery, or Merfolk
you control causes a permanent to leave
the battlefield, you may pay {u}{r}. If you
do, exile it. It gains Flash and becomes a
Merfolk in addition to its other types. You
may cast that spell from exile and use mana
as if it were any color to do so. Whenever
you cast a spell this way, it counts as an Instant.