msem-instigator

This spell costs {1} less to cast for each legendary creature you control.
Search your library for a legendary creature card with mana value X or less, put that card onto the battlefield, then shuffle.

Flying, double strike
Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)

Other Merfolk you control get +1/+1.
{1}{U}: Target Merfolk gains slipstream until end of turn. (Creatures with slipstream can't be blocked as long as only creatures with slipstream are attacking.)