Tideforce Elemental

The tap-or-untap activation is far older than this design (Master Decoy and its kin have been freezing attackers and blockers since the earliest sets), but bolting it to landfall is what gives this elemental its strange tempo. The body is fragile and the ability costs a mana and a tap to fire, so on its own it works like a slow, repeatable Pacifism: tap down the biggest threat each turn, sit behind it. The landfall clause rewrites the math. Every land that enters untaps the elemental, which means in a deck that plays two or three lands a turn (or fetches and cracks them) you are no longer choosing between attacking, blocking, and using the ability. You do all three. Tap to lock down their blocker, then play a land to untap and lock down a second creature; or play a land to untap the elemental and use the ability to untap your own tapped attacker for a surprise block. The card's whole strategic axis is the gap between "one activation per turn" and "one activation per land," and that gap is a deckbuilding decision rather than a property of the card. Feed it a steady flow of lands and it becomes a soft lock against creature decks, freezing a board two bodies at a time. Run it in a deck that drops one land and stops, and it is a 2/1 with an expensive party trick. Few cards tie a combat ability so directly to a land count.