Xavier Sal, Infested Captain

Two engines that eat their own inputs, wired into one three-mana body. The populate ability spends a counter from another permanent you control to copy a creature token; the proliferate ability sacrifices another creature to spread counters everywhere at once. Read them together and the loop reveals itself: proliferate builds the counter reserves that populate then cashes in, and populate rebuilds the token bodies that proliferate consumes. Neither half is a payoff on its own; each is fuel for the other, which is why this creature only pulls its weight surrounded by both counters and creature tokens rather than a single explosive combo. The "another permanent" and "another creature" clauses matter: Xavier cannot pay either cost with himself, so the engine only turns once you have outside material for it to feed on. The sorcery-speed clamp on both abilities is the real cost. Locking these to your own main phase means you cannot proliferate at end step to dodge sweepers or copy a token in response to a fight spell, and the tap symbol caps you at one activation per turn cycle unless you untap it. Those restrictions keep what would otherwise be a runaway counter-and-token machine to a deliberate, once-per-turn grind. The color identity is the tell for who this was built for: golgari's sacrifice fodder and graveyard recursion feed the proliferate side, while green and blue supply the token generators and counter density the populate side needs. Two mechanics that rarely share a shell, offered here as one closed system.