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1) The organization enjoys chaos
2) The organization is looking for power, and can remove the possession, saving the city and taking control with full support of the ignorant people
3) The leader of the organization is a demon
4) The organization is following prophecies from an ancient book.
5) Something terrible was going to happen to the city anyway (huge natural disaster like) and the organization decided to concentrate as much evil as possible in the city in order to wipe it all out at once
6) The high priest was close to discovering their secret, and they
needed to get rid of him. They never realized how vicious the demon would get
7) The organization was trying something completely different, messed up, and is now doing there best to solve the problem
8) The head of the organization worships a god of destruction and chaos, and is just following orders
9) The organization is one of scientists, and they are using the whole world as there lab. In this case they are studying [insert something like human suffering and its effect on the planes/whether there is a theoretical equality for bad/good, and measuring how a spike in the bad side could cause an equal spike in the good]
10) The organization is seeking the favor of the demon, and just allowing him to do what he wants to achieve some reward
I like number 5 a lot, maybe they used the deaths from the disaster to get enough souls to perform a ritual binding the fiends soul to the high priest's body while he slept or something. At some point I want the demon to fully gain control/manifest, ripping forth from the high priest's body, what fiend makes the best powerful mastermind?*stuff*.
5) Something terrible was going to happen to the city anyway (huge natural disaster like) and the organization decided to concentrate as much evil as possible in the city in order to wipe it all out at once
*stuff*
This is where all I can say is look through the monster manuals. They should have a lot of options. If you want your hero to have a chance, try to pick one with a CR close to what you imagine he will be.
Conversely, you can play it like an NPC character with a fiendish template or something. There is no reason to be too bound by what "officially" is a demon or not. Maybe even seeing many demon example will give you ideas for your own type :)
From your original description of what you're attempting to achieve it sounds more like the High Priest doesn't so much want to cause absolute chaos as tyrannical control. Tyrannical control is firmly in the realm of devils and subverting religions is also one of their long standing shticks. Considering that they're subverting a High Priest of Abadar (key part of his portfolio being merchants and wealth), might I suggest using a Cult of Mammon as your evil organization? Mammon is absolutely about corrupting people through wealth, or lack thereof. As the prices go up for supplies the greedy turn to Mammon hoping to generate more profits and the poor turn to Mammon hoping to make a deal that lets them live for just one more day, either way the wealth of souls coming to Mammon grows ever larger as he and his cult play both sides against the middle.