### Its body disintegrates and a lower-ranking modron evolves to take its place.

According to the Monster Manual, p. 224:

> If a modron is destroyed, its remains disintegrate. A replacement from the next lowest rank then transforms in a flash of light, gaining the physical form of its new rank. The promoted modron is replaced by one of its underlings in the same manner, all the way to the lowest levels of the hierarchy. There, a new modron is created by Primus, with a steady stream of monodrones leaving the Great Modron Cathedral on Mechanus as a result.