Stinging Lionfish

The reward here lives entirely on your opponents' turns, which is the tell for how this was built: as a creature that pays a controlling deck for doing what it already wants to do. The trigger fires on your first spell during each opponent's turn, so the price of admission is holding up instant-speed action every turn cycle, and the payoff is a tap or untap on any nonland permanent. That means untapping a mana source to slip in a second spell, tapping down an attacker before combat resolves against you, or tapping a would-be blocker during the opponent's turn so it stays down through your next attack step. The constraint is precise and doing real work: once per opponent's turn, on the first spell only, and never on your own turn. That ceiling is what keeps a two-mana body from warping combat and mana the way a freely repeatable instant-speed tapper otherwise would. And because Fish is only half the type line, the enchantment type earns its keep too: it feeds constellation and other enchantment-matters triggers while the tap effect quietly reads as a subclause on the reactive game you were going to play anyway. This is a small, pointed piece of design aimed squarely at the flash-heavy, hold-up end of the color pie, a creature that wants a full grip and an untapped blue source at all times.