Ancestral Devils were a demon race that existed only in dark and bloody tales and legends. Wherever there were demons, the legends of the Ancestral Devils were the strongest.
When Alm and Bruno arrived in the queen's universe, they went to the Divinestone clan's library and confirmed that the legends of this race existed there and were very similar to those from their home universe. They concluded that this race definitely existed in the past and was probably the progenitor of the current demons.
In these tales, a demon shrouded in an aura of death killed, tortured, and refined lives... basically a classic demon. Among other races, there were strange tales about humans too, depicting them as untrustworthy animals and their men as beasts who only thought about reproduction. Alm and Bruno never cared about these generalizations because they knew that a race doesn't define a person's character.
