Appropriate Dilemma?

[Synvallius]

For the finale of a grand (solo) campaign that I am running with a friend, I figured that the ultimate conflict will result in a severely difficult moral dilemma for his character to face, I'm wondering if the dilemma is "fair" in the sense that he won't feel cheated out of a good ending that ruins the whole game overall.
Some context: The character, Kodox, is a mage in a world where magic users generally have to make a deal with a demon in order to gain magical power. Kodox made a deal with a demon, Kael, who is trying to usurp the position of number two guy (Polmachus) for that world's version of the devil. In the final confrontation, Kodox is going to confront the Archmage who trained him, and the Archmage is going to go through a ritual that will allow for Polmachus to possess him, thereby giving the Archmage a chance of defeating Kodox, even if he doesn't come out exactly as he was before. Kael, seeing his chance for destroying Polmachus, will then attempt to forcibly possess either Kodox or one of his companions, all of whom are well liked by Kodox (who is neutral evil, but somewhat conflicted, and has a history of being occasionally sadistic but not necessarily wantonly evil). The dilemma will be that Kodox is either able to save himself and three of the four companions he has from possession, leaving the fourth to have their soul consumed by Kael; or he can save his four companions but will then be left defenseless when Kael attempts to possess him.
My question for my fellow Playgrounders is whether or not this is a fair end game scenario, or if it seems too rail-roady. If the former, all well and good, if the latter I'd appreciate some suggestions that follow the general idea of a confrontation between two powerful demon lords that can possess the bodies of mortals with limited difficulty.

[claypigeons]

Leave option C open. Let him be able to save everyone (with scars, physical or psycological) if jumps through enough hoops.

[Synvallius]

Any ideas for what those hoops might be? I was thinking that it might have something to do with how he has dealt with certain seemingly random situations that have cropped up (he seriously offended a a very powerful ex-demon lord who is working to keep Polmachus from coming back to the world, and I was thinking that this could somehow be worked in) the only problem with random events is that since the player has no clue what they might affect (or, indeed, that they are even significant) they would essentially end up making a choice accidentally without really being able to know the consequences of their actions. So any ideas supporting a third option would be welcomed so long as they make sense (i.e. not just a, "push the red button," type solution), and are not just fetch and return missions (my favorite, of course, but I don't want him to just have to find some "wand of magic demon resistance" that makes him able to win without repercussions). I'm very bad at thinking outside of my own personal box, so if there are any ideas they would be much appreciated.

[Theomniadept]

One idea I have; allow him to be possessed and attempt to fight the possession like a daily Dominate Person saving throw. See what he tries to do.

He could always Plane Shift to the heavens whilst possessed and see what that does. I don't think going to Celestia with a demon lord inside you is going to slip by the gods, so divine intervention would be almost immediate.

[Synvallius]

That was one thing I was thinking of, having some sort of divine (or at least extra-planar) intervention take place, but I don't know if that would have too much of a "Deus ex Machina" feel to it. And like said, I want this to be a finale with a dilemma, I don't want their to be an easy or even (necessarily) a right choice (pursuant to the campaign so far), and the main question I have (hopefully some people who have had experience with facing dilemmas of this sort in game can chime in and offer their perspective) is whether or not that seems like a good end game choice to have to contend with (me or someone I care about), or is it too constricting?

[rmnimoc]

That was one thing I was thinking of, having some sort of divine (or at least extra-planar) intervention take place, but I don't know if that would have too much of a "Deus ex Machina" feel to it. And like said, I want this to be a finale with a dilemma, I don't want their to be an easy or even (necessarily) a right choice (pursuant to the campaign so far), and the main question I have (hopefully some people who have had experience with facing dilemmas of this sort in game can chime in and offer their perspective) is whether or not that seems like a good end game choice to have to contend with (me or someone I care about), or is it too constricting?

As far as I see it isn't bad. You really don't need to add a divine intervention, or even create any obvious way to stop it. Just figure out the mechanics you want the possession to run on. Then leak hints here and there about how the possession works. When the end game happens, you will probably be surprised how well it turns out. Just make sure you come up with rules for the possession, and don't be arbitrary about it. Your friend is a caster, you don't even need to figure out how to win, just let him do the brain work and decide whether or not it is sucessful. Things you should figure out beforehand though, how does X spell or condition react with this. Things like:

Protection from Evil. How does it interact.
Mindblank.
Planeshifting to another place.
Suiciding after getting possessed, then getting revived.

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I've yet to see a player without the determination and creativity to think up escapes from even the most linear rails as long as the rules are consistent. Just make sure the tools exist, and he will either accept A, accept B, or choose C and carve out his own victory. It's a win-win, because either way the two of you have fun and get a great story.

[Synvallius]

Alright, thank you for the advice. Now, forward unto the challenge.