Silumgar Spell-Eater

The interesting part is where the counter hides. Face down for three, this is an anonymous 2/2: nothing about it announces interaction, and the tax rides on the flip rather than on a spell in hand, so the answer sits on the battlefield until you choose the moment to spring it. Turning a creature face up is a special action that resolves without the stack, but the resulting triggered ability is an ordinary trigger; it goes on the stack and can be interacted with, so a well-timed Stifle or a second counter can still make the tax fizzle before it lands. The flip cost is what keeps this from being a hidden Mana Leak on legs. Leaving that much blue untapped is the same tell any counter mage broadcasts, so a patient opponent reads the open source and either baits a wasted flip or simply pays the three and walks through the spell they wanted anyway. The tax only demands mana; it never refuses the spell outright, which is the softness you accept for pre-committing your answer. So the play is bluff management: hold the flip through a cycle, pick the one spell worth revealing yourself for, and know that if the opponent removes the sleeper first, the counter never triggers at all. When the flip does resolve, you get a 3/4 out of it, modest consolation for the total investment when the tax the whole card is built around doesn't stick.