What Varus Story Is Missing!: "Cast him in." -Kayn
Here's how I see it. In the comic, Valmar pulled a Soul Raver 2: Legacy of Kain
#on HIMSELF. Of his own FREE will.
There was no betrayal. No "Cast him in!" It was JUST like that, because he threw HIMSELF in! People want loss and tragedy in his story. All you really needed was some enemy characters to capture Kai and Valmar after murdering everyone and throw them BOTH in. With Valmar severely wounded, and Kai already 'dead'.
_"There was only one possible outcome: My eternal damnation. I... Valmar was to suffer the fate of traitors and weaklings. To burn forever in the bowels of the Pit of Pallas." _
https://www.youtube.com/watch?v=wUyFEn3kpTE
AND REMEMBER. We SAW multiple people burning in there!
https://i.imgur.com/F3yytzG.png
"Tumbling, burning with white hot fire I plunged into the depths of the Abyss. Unspeakable pain. Relentless agony. Time ceased to exist. Only this torture, and the hypocrisy that damned me to this hell. An eternity passed. My torment receded, bringing me back from the precipice of madness."
So all you really need is someone to throw the pair of them in there at the battle's end. THEN after burning for awhile (not centuries, or millennia -maybe like a couple years), he escapes with Varus fused with him hungering for vengeance. But Kai's soul is still with him and wants to turn him away from giving in. But it's not easy.
He realizes he's changed. And he hears a new voice in his mind.
"I know you Valmar.... You are worthy. Redeem yourself or...if you prefer AVENGE yourself. Use your HATRED to reave their souls. I can make it possible. Become my Soul Reaver, my Angel of Death."
https://www.youtube.com/watch?v=vlyDTEn32Fs
So that way the story is REALLY about Valmar with a sort of Devil on my Shoulder (Varus) / Angel on my Shoulder (Kai). And so this becomes an Id / Super Ego vs Ego. I think that would work well.