This requires a bit of explanation - due to technical problems, power loss, and attendance issues, we ended up having a few very short sessions followed by one big, entirely free form, boss fight. I did try to make sure that everything that happened in it was mechanically possible, but I needed to take a few liberties. I also do not have notes from a few of the small ones, and may be remembering certain elements wrong. Feel free to correct me if in the comments if you remember how it really happened.
As the party continued to make their way into the depths of the center, they began to notice a strange whirring sound. They followed the noise and found a teenage girl using airborne machines to saw away at the bars of a cage. Said cage happened to be suspended over acid. The party was able to get her out, and she introduced herself as Mary.
As the party attempted to make their way out of the dungeon, they were stopped by Father Myst and a cadre of his personal guards. Myst explained that Mary was the scion of the Chanton family, a group of powerful and very malicious mages that had secluded themselves in the Marshlands generations ago. Because of the evil perpetrated by her family, Myst viewed Mary as being beyond salvation and had left her to rot. He sent several of his guards down to deal with the party, who dealt with them in short order.
The party emerged from the center to find Alfreish in a state of complete chaos. The werewolves, city guard, and Myst's men were engaged in a three-way battle royale. Myst himself was commanding his forces from the bridge in the center of town. Sigurt climbed onto the roof of the Center to provide sniper support while Shamus and Denzai raced off somewhere to help out the werewolves and city guard.
After tearing through about a dozen of Myst's men, the remaining party members prepared to face the mad priest himself. Myst, by that point, had clearly lost what precious little remained of his sanity. He simply snarled as wings sprouted from his back and his sword caught fire.
After fighting for a while on the bridge, Myst retreated to the church's roof. The party climbed up after him. As Myst prepared to take flight once again, a well aimed rifle shot took out his wings. During the ensuing melee, the roof caught fire. Myst lost his footing an fell into the building, impaling himself on a candelabra.
The party looked inside, waiting for the priest to stop twitching. When they were satisfied tha he was dead, they made their way down from the roof. Just as they started to relax on the ground, the church exploded.
Myst rose from the ashes and yanked the candelabra out of his chest. His body began to change - he grew horns and a tail, his wings shed their feathers, and his teeth and nails elongated into fangs and claws.
Aisling and Mary were having none of this.
They immediately began to beat down on the demon. As Aisling fought, the smoke his tattoos emitted when he raged grew so thick that it encased him completely. His attacks grew more brutal than ever, culminating with his destroying Myst's body so thoroughly that only stray bits of bone and gristle could be separated from the mass of blood on the ground.
Aisling's rage subsided, and he subsequently passed out.
The party awoke the next day to find the town quiet once again. Aisling had no memory of the battle's finale. Shamus and Denzai decided to stay behind to help manage relations between the people of Alfreish and the werewolves, while the rest of the party moved on towards the forest.