Hmm!
Perhaps more info is needed but, I will run with some half baked Ideas of my own. From the standpoint of bringing about an adventure ( long or otherwise)
Ok I need to know if you guys would think this plausible
Yes plausible, too plausible. the main idea of devilish monk assassins seems over used ( I say seems, because in fact it isn't used much) I would say the combination of those three Ideas is mediocre ( Nothing against you, just the Idea)
I would take this to a new level, or another level anyway. For Instance You have a Monastary known to house Monks of Some Shadowy Order. The Characters are approached by a Celestial telling them that it is required of them to retreive an Item of Power that would be useful to their God from this Monastary.
( The Item I leave up to you)
The Monks turn out to be a group of nefarious non coporeal demonic creatures ( Not demons per se but their minions if nothing else) that have taken over the bodies of once at least decent monks which they are using to take out/destroy oposition to their masters returning
These psuedo demons are hording up powerful items that will be usefull to their master once he has again risen to his Physical form. ( Make him a True Demon, or Devil) And that these Items are an important node in the battle of good and evil being fought both in heaven, hell, and in the physical realm.
What the Celestial did not tell the Characters is that he is making a bid for Godhood, because he believes that the old god is not handling things correctly or that he is too weak to take on the evil of the Dark Lord that is rising.
To incorporate this into the adventure you may have him offer to make one of the characters his proxy once divinity he has gained.
And to make it even more amusing the Celestial ( Has chosen to bow before the Dark Lord once he has taken godhood) is corrupt, and has been working with the foul creatures of the Dark Lord to trick away or gain many Items of Power. ( as he is doing with the party)
At the end or near the end you could have a Bounty Hunting Celestial NPC come to the PC's and tell them the acts of the Demon, and the Lost Celestial.
If the character's fail in the quest the world becomes darker magic works strangely, and Clerical magic begins to falter.
If they succeed the world lessens in the amount of vile creatures that plague the world, Magic stablizes, and Clerical magic surges stronger ( so clerical damage such a Cure light wounds as cast by a cleric upon an undead do +2 extra damage) and the characters find a bunch of niffty Items which the Celestial takes away leaving them one or two Items of Minor Power.
None of what you said is going over the line, if there is a line.
If played right what I tossed out to you could bring a great long series of sessions. ( I took high level to mean nearing or already Epic level)
If you are interested I am already working on the Non coporeal creatures ( the Torpid) for a book I am writing. I would not mind tossing you the stats for the creature if you would tell me how it played in your campaign
