Nerissa, Trident's Wife

Prowess, islandwalk, vigilance
Whenever another Merfolk enters the battlefield,
draw a card, then discard that card unless you
pay {u}.
{3}{u}{u}{t}: Target land you control becomes a
tapped 5/5 blue Wave elemental with trample in
addition to its other types.
Discard a card: Create a 1/1 blue merfolk wizard.
Other merfolk you control have vigilance.