Tairais, Hourglass of Blood

Tairais enters the battlefield tapped.
{t}; You get a eternity counter. Any player may
pay 20 life to exile Tairias instead. You may only activate this whenever you could cast a sorcery.
At the beginning of your end step, if Tairais is in
exile, you may put Tairais onto the battlefield.
On your upkeep, if you have a eternity counter,
you win the game.