Grimoire Thief

A counterspell disguised as a mill spell, and the disguise is the whole strategic axis. Tapping it pulls three cards off an opponent's library into a face-down exile, and because you can read them, you spend the next several turns scouting the hand they haven't drawn yet. Each tap adds three more to the same exiled pile, so the longer the creature survives tapping, the deeper the file you're building on their deck. The sacrifice ability cashes in everything at once: flip every card exiled with the creature face up and counter all spells on the stack sharing those names. The two halves are mutually exclusive resources. The exile triggers on tap, not on attack, so a tapper or a convoke outlet can feed it without sending a 2/2 Merfolk into combat; but the counter requires sacrificing the body, so you either keep it tapping for more information or blow it up to deny known threats. Note the timing the payoff demands: it is a one-shot activated ability with no targeting clause of its own, so it does nothing until an opponent puts a spell on the stack whose name you've already banked in exile, and you must answer it there, before it resolves. You cannot pre-counter; you can only ambush a spell you've previously seen a copy of. That makes the payoff scale with redundancy: lethal against a deck leaning on four-ofs, merely informative against a pile of singletons.