The Dark One is a god-like entity and personification of several cosmic concepts; it is generally believed that it is only a mythical figure of legend and does not really exist.
The same rumors and stories claim that the Dark One, endorsed by the Narth and the Coven, had to make a decision to restore or destroy the RULE. This decision, so it is claimed, could have destroyed the Universe.
Richard Stahl once confirmed the existence of a Dark God-like personality, but he also said that this entity had left the plane of existence and sealed some sort of rift, preventing other entities to interfere with the Multi Verse. What connection the Dark Ones have with the Dark One is not entirely clear, but according to the Narth they were a Tech Level 12 Society that worshiped the Dark One and wanted his return to occur.
Another interpretation of the Narth Supreme's difficult-to-understand explanation is that the Dark Ones were a Race of Beings from another Dimension or reality and that the Terrans were the New Dark Ones. (Whatever he meant by that is not known)
However it is Stahl again who in an interview mentioned that the danger or the reoccurrence of Dark Ones was highly unlikely as his friend took care of them. (Again no further explanation given)
A Union Assembly request to interview both the Narth Supreme and Richard Stahl was voted down with the explanation that the Assembly and the Eternal Warrior had more important things to do than discuss myths and faery tales.