Glowing balls are a monster-only spell used by both druids (Keethrax and Guth'Alak), all varieties of liches, and skeletal kings, including the one faced in the lowest level of the Tomb of the High Kings.
In-game it appears as a blue "*", and on miss or hit, respectively, is described as
"You barely avoid a magical barrage." "You are hit by a barrage of glowing balls!"
Most sources of ADOM knowledge (and, confusingly, the death message) describe it as a "magic missile", although that does not correspond to in-game experience. In particular, characters with fire immunity take no damage at all from this attack, resulting in the message
"You resist the searing flames."
The barrage of glowing balls thus is a fire attack. PC-cast Magic Missiles are one of the few types of magical offense that cannot be resisted partially or completely.
Without fire resistance the damage depends on monster type:
Guth'Alak did around 20-30 damage when testing, while a normal lich did 40-50.
The glowing ball attack does not require a direct line like a bolt would, though corners might be used to block it; it seems like this spell follows the same path as using any kind of missile (or casting the improved fireball spell) would. Monsters standing on the course between the character and a potential caster of glowing balls seem to stop this attack even if the monsters in question are immune to fire. The same holds true for the character him- or herself - a monster behind you in the trajectory of the glowing ball can never be hit.
Glowing balls never cause worn or carried inventory destruction, although they do seem inclined to melting beeswax in ears plugged with it (on a test this happened two out of five times the character was hit, even though he resisted the glowing balls' damage). Just in case, look out for glowing balls-wielding monsters when facing the banshee.
"The bee wax in your ears melt due to great heat."
Probably this could be best described as a fire arrow cast by monsters and aimed at the character's face. It also does not melt ice bridges.