Agate Instigator

Red's pingers historically taxed you for attacking or tapping; this one keys off the moment a creature enters under your control, converting go-wide sequencing into a slow drain that hits every opponent at once. It is a payoff, not a source: it makes no tokens on its own beyond the single copy Offspring buys, so it lives entirely on the traffic the rest of your deck generates. That Offspring copy is the wrinkle that lets it prime its own pump. Pay the extra and the 1/1 that arrives is itself an "another creature you control enters" event, so the card immediately stubs in one point of damage while leaving a body behind to keep the counter turning. The 1/3 is the tell about how it wants to be built. It is not attacking anyone; it sits back and taxes the table while the board floods out beneath it, banking the reach you would otherwise have to earn across a whole combat step. Every mana dork, every token, every blink and recursion trigger becomes a ping, which rewards the exact kind of deck that already wants to empty its hand and rebuild each turn. The damage is small per trigger and only ever aimed at opponents, so it reads less as a finisher than as a clock that runs on the noise a creature deck makes anyway.