Dagoth Ur was already a traitor working with Shor and the Nords, and the role traditionally played in history by Voryn Dagoth as Nerevar's right-hand man is filled by Alandro Sul.
So it literally says that Voryn Dagoth and Alandro Sul are different people.
In the Dagoth Ur that we meet, with the three-eyed visage of Sul and the memories of brotherhood with Nerevar, I think it's a fusion of the two entities.
I don't recall any source that proofs that Alandro had three eyes.
It's extrapolated from the Secret Song of Wulfharth Ash-King. In it, Dagoth Ur was already a traitor working with Shor and the Nords, and the role traditionally played in history by Voryn Dagoth as Nerevar's right-hand man is filled by Alandro Sul. In the Dagoth Ur that we meet, with the three-eyed visage of Sul and the memories of brotherhood with Nerevar, I think it's a fusion of the two entities.
Nerevar had many friends. And they all would have memories of brotherhood and friendsip with Nerevar. Alandro, Voryn, Vivec, Almalexia, Sotha Sil, even
Dumalakatoshor Dumac the Dwarf-Orc. Wouldn't you say they all are one entity?
If as it claims, Nerevar is the son of Boethiah, and Alandro Sul is son of Azura
Beautiful titles, nothing more. But...
"Sons and daughters of" should be read as associates of/associated with, especially insofar as this association was a conscious choice.