Moonstone Harbinger

A tribal lord that keys off the life total moving rather than combat or casting is a small but pointed design choice: any life change during your turn, gain or loss, in either direction, flips your Bats into deathtoucher swarm. That "gain or lose life" clause dovetails with how a Bat deck already wants to play, draining opponents while feeding its own engines a point of life at a time, so one incidental drain or one painland activation arms the whole board. The restriction that keeps it grounded is that the pump fires just once per turn, so several separate life changes in a turn do not cascade into a runaway board and the card stays an aggressive lord instead of a combo piece. Its own body carries the idea past the ability: deathtouch on a 1/3 flier survives most combat and holds a defensive lane while the offense builds, so even on turns the trigger whiffs the creature is doing tempo work. The +1/+0 is small per point, but spread across a wide board it turns a flock of tiny fliers into attackers no one can block profitably, every one of them a deathtouch trade waiting to happen. The archetype's natural rhythm of nickel-and-diming a life total up and down is exactly the fuel this asks for, which is what makes it the anchor of a swarm rather than a fringe curve-filler.