Kind of foolish of Brian not telling him.
Maybe he wants to keep the two strongest entities fighting each other so that one doesn't become supreme.
Or maybe Aridon knows already and simply lets him do as he pleases, as long it doesn't threaten Arkain.
Not that Aridon could do anything right now, but it would have been nice to know.
Maybe Aridon could do something.
Remember that Largoth had to be shielded in order to prevent Aridon from finding him, so Largoth knows that he stands no chance against Aridon. Also, Largoth used paralysis on Veringar and the undead, most likely because they could actually kill him if he didn't use it.
So basically, Largoth can be defeated.
Lisara, while powerful, was imprisoned by her own kind. The Undead have managed to defeat several demons across the centuries, and we did defeat Ornassion, one of the top demons, and he is now under Aridon's control.
So Lisara can be defeated.
This leaves the Dark One without his two top servants.
The Golden Guard is no more and the Empire lost many forces during the war, so the remaining Imperial forces can be overpowered by the undead. Plus, the undead forces have been getting stronger over the centuries, and now they have Rahandir, the Bone Lords, Ornassion, etc...
The Dark One was defeated single-handely by the Demon God. And Aridon killed the Demon God. So I have little doubt that Aridon, backed up with his most powerful servants, can kill the Dark One.