The Sunset Gates

Military action

A party of high-level adventurers is contacted by the Riftwalkers, notably Lander and Lady Whisper, and asked to fly to Sunset, former capital of Dusk, and shut down three portals that have opened there, allowing fiends to pour forth into the material realm. They travel to Sunset, defeat many powerful foes and close three planar gateways, including one anchored in the Abyss itself.
At the third gateway, the party refuse a deal with the archdevil Hutijin, but in the process allow him access to a powerful soulstone. Only the intervention of a powerful celestial, Calisto, prevents catastrophe.

A party of high-level adventurers is contacted by the Riftwalkers, notably Lander and Lady Whisper, and asked to fly to Sunset, former capital of Dusk, and shut down three portals that have opened there, allowing fiends to pour forth into the material realm.
The assembled are Elli, a chronurgist wizard, Ezri Snowblossom, a deceptively powerful fey, Tywn, a member of the elite First Ilfrindori Royal Scouts and master assassin, Gawain, an extra-dimensional knight bound up with the Circle of Blades, Chezzerat, a sorcerer whose very body has been wracked and warped by the chaos he controls, and Aire, a man touched powerfully by the divine.
At the briefing, Lady Whisper expressed her frustration at being unable to go with them, and also that her scrying was partially being blocked. Twyn did a little careful snooping, and Ellis suggested that they just teleport.
The party flew in on board the Silver Fury, the dwarven airship captained by Grovkin MacHammer, fending off waves of flying undead and, notably, the lich form of the former archmagister himself, Erasmus the Sun-King, riding a blue dracolich. Though they were able to discorporate the liches and disable the first portal, they made an enemy in the form of Erasmus, whose spectral forms continued to pursue them across the city.
They find and destroy the first portal inside an old manor house, noting the strange mix of magic used to create these gateways-- a fragment of Asmodeus’ ruby rod was anchored inside a sphere of pure, raw primordial chaos, all bound within an archway of blackened godbone.
After taking a short rest, they sneak their way across the city to the old theatre district, where the next portal was. They dealt with its guardians-- a bound Goristro and the strange fiend known as a Herald of Darkness-- before discovering that this portal was anchored on the other side, and they would need to cross into the Abyss to destabilize it. The party did so, plunging forth into the smoky realm of layer 303, the Sulfanorum, where they did battle with the horrific Herald of Blood as well as numerous cambions and vrocks. As they destroyed the portal keystones and nervously watched the portal collapse, preventing their return journey, Gawain simply said “Trust in me, friends. Join hands.” and plane shifted them back to Faern.
The party arrive at the base of the Duskwatch Tower, once Erasmus’ personal laboratory. At the base, they find a colossal soulstone, an artifact that contains hundreds of thousands of souls-- most of the souls belonging to the undead now ravaging Dusk, as it turns out. Outside the next portal, they meet the archdevil Hutijin and are offered a deal-- break the summoning circle binding him here, or else he will drag their souls, and everyone they’ve ever known, to the Hells personally. His lieutenant, a pit fiend named Placeholder, gives them a weird tour, giving them a chance to talk it over. They decide to kill them all for threatening them, though Elli at least talks about considering the deal.
During the battle, they accidentally disintegrate the magic circle binding Hutijin, allowing him to make a break for the soulstone at the base of the tower-- and the massive repository of soul energy contained within. Only the timely intervention of the celestial Calisto prevents catastrophe, as he teleports the party to safety as hell plunges down to stop Hutijin's feast.