But... Part 2.
Tristan found the big soulstone in its protective casing sitting there. he could have flown off. he could have ignored it. He could have turned himself into the authorities and accepted judgment.
Instead he touched the stone, and the balor spoke to him. "Release me, and I will reward you greatly!" Tristan, without much thought, said, "Hey, cool -- no problem! I hate this place anyway." So he starts taking a hammer to the stone. No one has yet arrived from the angel/monks to see what had happened. After a few rounds, he breaks the casing with all of its protective seals. Then he lifts the sphere and smashes it on the ground. There is a huge flash of light. Even the party members on the airship see it and FEEL it as the wave passes the boat; it's like a huge bright bomb just went off.
Tristan finds himself standing next to a 12' tall balor demon. It grabs him and says it will have him watch the destruction of the city as a reward before it terminates him. What? The demon lied? Tristan is indignant. How rude! Demons can't be trusted, not ever! Even his own demon lord master must be a flake.
The demon smacks the ground and Desecrates the area. The Desecration is a quarter mile wide and even encompasses the Gate. It creates a permanent zone where evil creatures are stronger and good creatures suffer. It then starts summoning crap and killing whatever is nearby. The entire city is under siege.
In its distraction, Tristan manages to break free and plane shift. The angels could have followed him before, if he had tried that, but right now they have worse problems. He goes to the fey and converts back to their cause... but his time with the party is now over (because no one wants to adventure with him because of the crazy evil stuff he just did).
On the airship, the party sees a huge circle of red flame leap up in the city they left, turning to sicking green fire (the Desecration). The entire city is going up. The battle there must be hot and fierce. At best, they can just hope that no one knows they were involved in that cataclysm.
Sumiko is repentant as the enormity of what she just contributed to burns itself into her brain. All the rage and arrogance and bitterness, of toying with power, but this is where it ends up. One of the party takes her under his wing and promises that he'll help her change. And at the moment she is open to that.
[I'm considering playing her in the OTHER direction now, with the alignment conversion... almost too good, annoyingly so. Lol. Whatever would seem fair but also funny.]
her story in this sense is kind of like Ged from "A Wizard in Earthsea":