Taniwha

A 7/7 trampler for five mana would be a bargain if it did not strip your mana base every time it shows up to fight. The upkeep trigger phases out every land you control, so on the turn the serpent is present, you owe something close to an Armageddon to your own ability: float mana in response if you like, but mana empties at the end of the step, so you cannot carry it forward to your main phase. Your lands return before your next untap, but the turn the serpent arrives plays out with no lands. The creature's own Phasing is the release valve. While it is gone from the battlefield, the upkeep ability has no source and never fires, so the land tax only bites on the upkeeps when the serpent is present. That sets up a strict two-turn rhythm: the turn it phases in, your lands vanish and the body attacks off no land mana; the turn it phases out, your lands stay and you get a window to rebuild, but the serpent is unavailable for combat. The drawback is symmetrical to the benefit by design. The same temporal sleight that keeps the serpent out of removal's reach locks your own resources in the same closet, and the trigger that empties your mana is one you skip every other turn for free. The puzzle is not survival but sequencing: learning to spend before the lands disappear out from under you.