Anytime ten or more skeletons are found together and fought together, when four or more have been defeated in a way that doesn't entirely vaporous their substance — as when a cleric destroys undead — there is a 25% chance those four will form into reborn one, a 4+3 HD specimen with 27 HP with the same treasure as other skeletons.
The reborn one forms itself of the scattered and shattered bones of other skeletons, and the formation of a reborn one is usually evidence that the particular site on which it occurs is, or at least once was, a scene of great evil.