Tekuthal, Inquiry Dominus

Doubling counters is old ground: Doubling Season and its kin ask a deck to build up counter production and cash it in for exponential returns on whatever permanent happens to hold the counters. This design doubles the action, not the object. A proliferate replacement effect turns every proliferate event into two full passes across the whole board, so the value scales with how many separate proliferate sources you fire rather than how many counters are sitting on any one permanent. That framing rewards a particular engine: cards that proliferate incidentally and often, each of them quietly pulling double duty while the Horror is on the battlefield. The self-protection ability sets a price that keeps the payoff from running away on its own. It demands both a mana payment (with the Phyrexian pips letting you pay life instead of blue) and the removal of three counters from among your other artifacts, creatures, and planeswalkers: a permanent that is none of those types cannot contribute. The same resource that fuels the board pays the insurance premium, and the counter cost is fixed regardless of how you settle the mana. A 3/5 flier is durable enough to stick and doubles as a real clock, which matters because the card is dead weight until you have proliferate to double. This is a build-around in the strictest sense: not a card you splash for a marginal edge, but one that reorients an entire deck around how many times per turn you can proliferate.