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.)

Kinjiru Assailant has haste as long as an opponent controls a nonbasic land.

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.)

Marin Polgek, Academy Heiress has hexproof as long as you control three or more enchantments.
{1}{W/U}{W/U}: Create a token that's a copy of target enchantment you control.