Leviathan enters the battlefield tapped and doesn't untap during your untap step.
At the beginning of your upkeep, you may sacrifice two Islands. If you do, untap Leviathan.
Leviathan can't attack unless you sacrifice two Islands. (This cost is paid as attackers are declared.)
10/4/2004 You don’t have to pay the untap cost if it is untapped outside the upkeep phase or is untapped by an effect.