Sleth'not, the Everdeep

Defender.
{b}: Regenerate Sleth'not, the Everdeep.
Creatures your opponents control cannot attack you or a planeswalker you control unless they have flying or islandwalk.
Sea Monster spells you cast cost {1} less to
cast for each Island your opponents control.
(Serpents, Leviathans, Krakens and Octopuses
are Sea Monsters.)