There is a humanoid outsider that follows the Alienist around, observing the Prime Plane and commenting on it, on how it wants to actually be there, to find out what 'touch' is, or get to try and experience 'faith' or 'emotion.' Give this entity a mind and heart of stone, with only one want you can even understand: it wants to take your body from you.
So as the game progresses, the alienist makes secret rolls, matching his will against the creature as it periodically tries to rip his mind out and take the alienist's body for its own. As time passes, these rolls go from laughably easy to nearly impossible.
So the Alienist starts confident, sure that this new source of power for himself is no threat, and slowly loses that surety. Slowly, he becomes more frantic, distracted from the parties goals as he worries about how to preserve his mind from this eldritch being that stalks him, tirelessly trying to steal his form. Near the end, the player should start displaying strange behaviors, like eating knowledge (consumes written texts to learn from them, yet can also now chomp on stone, so long as it is a tablet with writing or such on it)
And eventually he regains his composure. Sure, he does the wierd behavior traits even more now, but he has an interest in the quest again, and when asked he tries to keep his stranger quirks quiet.
And then, some time later, the party, for whatever reason, needs to travel to a plane very far away, some nameless and nebulous place, to fight this euclidean being that threatens to do... something. You aren't really sure.
Only when you get there, the creature in question is half crazed, barely able to move with more grace than an infant new to its form, that, if given the chance, slowly croaks out something like:
's...ss...ssooorrryy. Iiiiiiiit beee-bee-beeeeeaaaat mmeeee. Hheeeeeeeelp meeeeee!'
And that is when the Alienist's behavior changes a very great deal.