Timesifter

A land card has a mana value of 0.

Each player must exile the top card of their library at the start of each upkeep step.

Each time there's a tie for the highest cost, only the players involved in the tie continue.

If a player fails to exile a card, they can't take the extra turn.

If multiple extra turns are created in a game, the most recently created extra turn is taken first.

If two or more Timesifters are on the battlefield, keep a careful record of which players get which extra turns. With two Timesifters on the battlefield, two extra turns are created for each turn taken.

Remember which player would have taken the next turn if Timesifter's ability hadn't triggered the first time. After Timesifter leaves the battlefield and all extra turns have been taken, that player takes the next turn.

In Two-Headed Giant, triggers only once per upkeep, not once for each player.