Mystic Sanctuary

The whole engine hangs on a single word in the enters clause: untapped. Meet the four-Island threshold and the land arrives ready to buy back an instant or sorcery straight to the top of your library; miss it and you get a tapped Island and nothing else. That conditional is the entire balancing act, because the recursion is uncapped in every other way. It targets any instant or sorcery, costs no mana beyond the land drop, and refills the same spell turn after turn. The obvious partner is any effect that returns the Sanctuary to hand or the battlefield: bounce a copy of itself with the flurry of untap-and-replay tricks blue has always hoarded, and the tapped-unless clause stops mattering once you can guarantee the fourth Island. What makes it genuinely dangerous is that it recurs the spell that recurs it, or the counterspell that protects the loop, so the buyback pool includes the very tools that keep the engine going. That self-referential quality is why it draws so much scrutiny in eternal formats: a land that reloads your best interaction for free, on a manabase you were already going to run, asks very little and gives back a great deal. The Island subtype is not decoration either; it feeds off fetch lands and duals that count toward the threshold, which is how a card with a strict-looking condition ends up being trivially satisfied by decks that wanted blue mana anyway.