Even for a five-color land, Tarnished Citadel is remarkably flexible, even more so than its competition
Every Adventures in the Forgotten Realms Collector Booster contains 15 Magic: The Gathering cards and 1 traditional foil double-sided token, with a combination of 5 Rares and/or Mythic Rares, 25 Uncommons, 47 Commons, and 1 Land card
With enough mana, a player can just control the rest of the game, and theres no worse way to lose than to yourself
It might even see Standard play in toxic decks with Prologue to Phyresis and similar cards that never quite seem to finish the game
In fact, it's likely we won't see it again, and future Universes Beyond products will use their own, similar keywords