Shelldock Isle

The Hideaway mechanic always carried a tension: you tucked away a card for free on a tapped land, then waited for a condition that might never arrive. Most of the cycle keyed off something visible at the table (a creature dying, a permanent count) and was easy to play around. This one keyed off a library reaching twenty cards or fewer, and crucially, it counts any library, yours or your opponent's, whichever empties first. That single number rewrote what the land was for. Fill a deck with cantrips and self-mill and you can turn the exiled card into a free cast well before a normal game would reach that count; in a grindy matchup where the opponent draws and digs hard, their deck can trip the condition for you. The detail that let this land outlive the rest of its kind is the activation timing: a tap ability resolves at instant speed, so you can fire it on the opponent's end step, sidestep sorcery-speed answers, and untap with the threat already on the board. Because the exiled card is played without paying its mana cost, the payoff is uncoupled from the spell's printed price entirely, and the natural target became whatever expensive, game-ending thing you most wanted to cheat. A blue source that does nothing for most of the game and then hands you a no-cost cast: the whole design lives or dies on hitting that library count, and a deck built to thin itself can get there astonishingly fast.