Sanctum of Ugin

A land built for one deck, and explicit about it. The colorless mana it taps for is filler; the real text triggers only when you cast a colorless spell of mana value seven or greater, which in practice means the enormous colorless threats a ramp deck is trying to land. At that point it eats itself to tutor up another colorless creature straight to hand, no penalty beyond the search and shuffle. The structural cleverness is that it rides on the cast trigger, not resolution: even if your giant threat gets countered or exiled in response, the Sanctum has already cashed out for the next one. That makes it a chaining engine for the deck that ramps hard, lands one huge body, and needs to refill the curve when the first wave gets answered. Few tutors ask so little: no card, no real mana, just a land and a spell you were casting anyway, with the catch that it only ever finds another creature, never the ramp or the answer or the second land drop. Outside a dedicated colorless-fatties shell it does nothing but produce a single colorless mana, which is the honest price for a tutor that fires off the back of a spell already on the stack.