The hail elementals are born inside the clouds of Nimbus Ascent, deep in its magical, storming core. They look like a loosely-bound of large hailstones in a vaguely humanoid form, bound by filaments of purplish lightning. The hailstones vary in size from small gravel sized ones, to golf ball sized, with a few melon-sized oblate spheres of ice forming the head, fists, heart, and feet. They are a natural formation of the elemental energy of the Nimbus Ascent.Hail elementals are like an immune system for large clouds, aggressive to forms of life they perceive as invaders. One can not settle on the Nimbus clouds, since the elementals will keep coming, generated somewhere in the clouds core. A nimbus is something like a coral reef, with a number of “accepted” symbiotic organisms that live around it, protecting it (or just their territory), crowding out potential parasites, etc. The large clouds are effectively each their own ecosystem. Destroying the magical “marrow” deep within the cloud where the hail elementals are produced could spell the death of an entire ecosystem. Or new safe places to settle, depending on who you talk to.
The Starslayers may have more information about them for the curious.