Walk the Aeons

Extra turns have always come with a tax meant to keep them from chaining trivially, and the buyback clause here is the most literal version of that bargain: the spell will return to your hand, but only if you tear three Islands out of your own manabase to do it. That is a self-inflicted wound on a card whose entire purpose is to take more turns, and the tension is deliberate. Cast it for full price and you net an extra turn at the cost of six mana; cast it with buyback and you keep the spell forever but watch your land count crater, so each loop demands a way to refill Islands faster than you sacrifice them. The design refuses to give you a free engine; it hands you the engine and the price tag together and asks you to solve the gap. That puts it in a different category from the extra-turn spells that simply exile themselves or cap at "this turn only." Those say no permanently. This one says yes, repeatedly, as long as you can manufacture the Islands to feed it: a puzzle about ramp and recursion dressed up as a time-magic sorcery. The reward for solving it is open-ended turns, which is exactly why the cost is land you can never get back through the spell itself.