When Hela unleashed the spirit horde from the Abyss, it was not just the souls of evil men, orcs, and other humanoid races which escaped. The souls of many evil dragons also found their way back to the mortal realm. Most found themselves as wanderers without form and were taken to Hela’s secret realm, but a rare few managed to locate a dragon corpse, returning it to life as an undead. Their corpses vary from near-perfect specimens to virtually skeletal forms.
Their breath is a stream of necromantic energy, which withers living flesh and ages solid matter to the point it crumbles to dust. Unlike with a necromantic spell trapping, undead dragons do not heal wounds through their breath. Regardless of what form they took in life, all Undead dragons share a common template. Although dead, undead dragons continue to grow in size and magical power through the ages.