Whenever Agent Frank Horrigan enters or attacks, proliferate twice. (To proliferate, choose any number of permanents and/or players, then give each another counter of each kind already there.)
Cast Agent Frank Horrigan as a resilient 7-mana beater that snowballs counters across your board, then attack to proliferate twice each turn while being effectively unkillable in combat. You build a wide or tall counters board (+1/+1, charge, oil, infect/poison) and let repeated proliferate convert small advantages into a runaway lead. Many builds simply ride Horrigan's trample plus proliferated counters to lethal, or use him as an engine that ticks up planeswalkers, saga-likes, and toxic creatures.
Indestructible while attacking makes him a near-unblockable, board-wipe-resistant threat
Proliferate twice per attack compounds every counter source you run, scaling explosively
BG access to ramp, recursion, and the best counters payoffs in the format
Doubles as both a finisher (trample beater) and an engine (counter multiplication)
Proliferate also advances planeswalker loyalty, sagas via lore, and poison/infect kills
Seven mana means he often arrives turn 5+ and recasting after exile/bounce is painful
Proliferate does nothing if you have no counters on board yet
Vulnerable to exile, bounce, -X/-X, and edict effects since indestructible only helps in combat and only on his own turn
Black-green lacks interaction with stack/combo and struggles against fliers
Telegraphs the wincon, drawing removal and chump-blockers
Legendary Creature — Mutant Warrior
