Thief Dragon was not a true dragon, but rather the legendary Corpse-devouring Worm Emperor.
This creature, like the Dragon Sturgeon, belonged to the Mutant Species True Dragon, with pure True Dragon Treasure Blood flowing within its body.
However, unlike the Dragon Sturgeon that couldn't cultivate, the Corpse-devouring Worm Emperor could.
Speaking of the Corpse-devouring Worm Emperor, it was necessary to mention the Corpse-eating Worms which, as the name suggested, thrived by devouring corpses. Legend had it that they were the descendants of a True Dragon and an Alien Clan from the Mythical Era, whose dragon blood had become so diluted over countless generations that it was nearly imperceptible.
Naturally, it wasn't always like this; there were times when a mutation would occur among the Corpse-eating Worms, resurrecting the diluted dragon blood completely and transforming it into pure True Dragon Treasure Blood.
