Submerge

Five mana to put a creature on top of its owner's library is a price nobody was ever meant to pay; the printed cost is a fiction, scaffolding for the condition that matters. Control an Island while an opponent controls a Forest and the spell becomes free, which is to say it exists to punish green for being green and reward blue for staying on plan. The era produced a whole batch of blue spells built around blue cashing in its own lands (Gush, Daze, and Foil all let the deck spend Islands as currency), but this one narrows the gift to a single color war, with the lands across the table casting the deciding vote on whether you got a gift at all. As an instant, it waits in hand until a creature needs to vanish, and the free mode makes holding it costless: you commit nothing to keep the answer live through the opponent's turn. The bounce mode is a quiet cruelty. Stacking a creature on top of its library does not just clear a threat, it eats the owner's next draw step, so recovery costs them a card they would otherwise have seen. That is a strictly worse outcome for them than ordinary hand-bounce, and the reason the effect was hung on a mana cost steep enough to guarantee it stays unpaid. The whole design is a wager on the board: backbreaking in the right standoff, dead lumber anywhere else.