Styx Leviathan

Islandwalk (This creature can't be blocked as long as the defending player controls an Island.)
Undying (When this creature dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.)
Styx Leviathan can't attack unless there are five or more Islands on the battlefield.