Horn of Gondor

Most army-in-a-can artifacts fix the payout: they hand you two, three, five bodies regardless of context. This one keys X off the number of Humans you control, which makes the enters-the-battlefield Soldier a seed rather than the point. Every subsequent activation reads whatever Human count you have assembled since, and because the tokens the Horn makes are themselves Humans, each firing raises the ceiling on the next one. Left unchecked across a few turns the numbers stop being additive and turn exponential, a compounding loop that a one-shot token spell can never replicate. What gates it is the mana: to cast, then and a tap every time you fire it, so the loop is throttled by how much you can pour in per turn rather than by any per-use cap. That per-turn tax is the reason the engine stays fair; it demands a board that survives long enough to be milked instead of a single explosive turn. It slots naturally into a tribal frame, where the creatures already filling out the curve double as the multiplier, and the lone Soldier on entry reads as a deliberately small down payment on a payout that only arrives if you can protect the artifact and keep feeding it.