Okay, I wasn't really feeling this until I read the explanation. At first I thought it was a serviceable idea, touch of flavor to throw in, maybe a mid-level villian, but with the added fact of the drow, it has much more potential. If drow exist and are known, and he is found out, what implications will it have? Will the authorities believe he is outcast, or think he is a spy? Will they kill him immediately or imprison him? If drow exist but are not known, what will they make of him if they discover him?
If I were to use this, I think I would change the imprisoned being from a demon to an angel.
The reason the monks imprisoned it would stay the same, they wanted his angelic knowledge. They could have been evil monks looking for a way to defeat Good, or good (well, neutral probably) monks that are just after knowledge in general.
The angel kills the adventurer to get out, and then fights the other adventurers because it is either insane after a millenium of imprisonment, or because it truly did need someone to sacrifice themselves to get out, and the fight afterwards was just a reaction from the adventurers who were angered at losing a friend. After all, just because a being is "Good" does not mean they have are nice, and if the death of one person, or one adventuring party, means they can get back to fighting the Greater Evil, then so be it.
The best part of this is that all the standard powers and abilities (assuming a system like D&D) that the party has will be useless against an angel.
I am a novice GM (or rather, I consider myself novice, since my game has undergone two edition changes since I last GM'd, and even then I used a published world), and I think this article will help me greatly. I have read the other worldbuilding artlicles here (and elsewhere), and there almost too much advice to remember. These four maxims are much easier to keep in mind, and seem to be distillations of many other pieces of advice.
The PCs are all Apprentices in their various fields. One day, it is discovered that their village/town/etc is under some form of iminent threat (advancing horde, dragon seen nearby, James gang riding in, whatever). The town leaders get together, and decide that someone needs to go get help from regional authority (the duke, the marshal, you get the picture). They don't want to send any members of the local constabulary since they will be needed to keep the peace/defend the village, so they ask the Masters to send their best Apprentices to ride for help. The logic being that the best students can be trusted with the job as well as being competent to do it.
The fun comes in when the players decide if that's true. Maybe they are the Master's best pupil. But maybe, they are the worst, and the Master wanted them far away so they didn't mess up the defense of the village. Or maybe they have a relationship with the Master beyond what simply teacher-student. It could be romantic (either condoned or illicit), or it could be familial. Whatever it is, the Master sends the PC for help to get them out of the village and out of danger.
Hopefully, the party will at least have enough top grade students to fullfil their duty (and survive), but it's the other students that will make the job interesting.