I don’t really see a reason why would Orpheus side with Emperor. Enslaved Prince is the only thing Emperor is bringing to the table.
What alternative would Orpheus have? Fight the party and get killed? He's outnumbered 5 to 1 (party of 4 + emperor). Even if Orpheus were to win, he'd be so badly weakened that the Netherbrain would have an easier time carrying out the Grand Design, having lost potentially 5 people to side with. Orpheus is trying to save his people from Vlaakith here. Getting killed from being outnumbered 5 to 1 isn't going to save the Githyanki, nor is allowing the Netherbrain to complete its objectives (which would bring the Githyanki back into slavery).
As for the Emperor, he brings a lot more than you might think. Keep in mind that the Emperor is still another ally in the fight - and if the player has not chosen someone in their party (themselves or Karlach) to become a mind flayer, he's the only person who can bring the Netherbrain to heel. It's clear that Orpheus himself doesn't want to become ghaik (he begs you to do a "mercy killing" if he has to transform to become ghaik). Every bit counts when the odds are against Orpheus.
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The way I see it, it gives the player options. If you want to RP siding with one or the other, there should be an option to side with one or the other, which is the status quo.
But if you want both and are able to pass a dice roll, it gives the player options to try and roll for both.
