Going off of Crimson King's post - I think the Otherness would have no more use for Rasalom, were Earth to be completely overrun and taken over by the Otherness. Both Rasalom and Glaeken have served on this planet (no mention of them fighting on other planets), as proxies to the Otherness and the Ally, respectively, and as the battle would have been won by a singular side, the forces would no longer have a use for their proxied representatives.
Would Rasalom be stripped of his powers, and be allowed to "live out his days as a mortal" (as was Glaeken, when the Ally perceived Rasalom was gone)?
The Otherness, it seems, cares nothing for those it uses (as does the Ally), so why would Rasalom think he had a position when the Change was completed? I feel that Rasalom, would the Otherness win, be treated much as Rasalom has treated those like Ernst Drexler (The Order's Actuary) - used until no longer needed, then tossed aside without a second thought.
Rasalom's hubris has been his downfall on multiple occasions, so this, I can expect, would be the ultimate irony.