"But if there must be an end, let it be loud. Let it be bloody. Better to burn than to wither away in the dark."
-Mike Mignola
With their followers whipped into a fighting frenzy, and Ur-Balat successfully brought to their side, The Green Dragon Brotherhood plans the final assault on Crow's Keep and the Grey Guardian.
The party has gathered hundreds, if not thousands, of followers, all of whom are dedicated to following them into battle to reclaim their city and install Greer the Betrayer (aka the Bastard of the Moors) on the throne. And the Brotherhood sails them across the water, marches them up to the city gates, gives them a dragon and says, "Okay, catch you guys later..."
Because the Green Dragon Brotherhood never plays it straight.
They quite rightly recall that they once slipped out of Crow's Keep unseen, so why not slip in the same way? As their forces land under the cover of darkness, the Brotherhood sails to the sewer drain from which they left several years ago, and after Kraz Formalhaut disarms a newly-placed trap, they let themselves in ( Lady Rinwolde thought they might sneak back, even if the GM some-fucking-how didn't and had to make up the next hour of gameplay as he went).
They soon find themselves noisily unlocking the trap door back into the castle's dungeons, which alert a small watch of Lizardfolk, who have been guarding this entrance, it seems, for this eventuality. They try dropping flaming oil on the party, all of whom dodge it but whose fire-resistant suits from In'thok would have protected them anyway. Then it's a simple matter of dispatching these poor schmucks, one of whom Cleansing Waters captures with his bionic harpoon. This poor Hrassk reveals that the Guardian and the Order of the Iron God are holed up in the throne room, even as the battle rages above.
With Bess Ashglade and a few fellow travellers bringing up the rear, the Brotherhood ascends to the throne room and finds that the guard was telling the truth. The Order stands ready for battle, their skin grey and their eyes blank, while the Guardian sits on the throne, silent and unknowable, the bones of King Uldred and his vizier piled about it.
Chaos ensues.
Kraz Formalhaut, the man of science, again dips his toes into magical waters and uses his Twin Storms Spear to send chain lightning dancing from one corrupted knight to the next, dropping several of them instantly, with small grey oozes leaking from the corpses.
The knights close the distance and it's steel against steel, even as arrows rain down from the level above. Suddenly, a chill wind blows through the room, and Lord Calder is somehow there, corrupted as the rest, swinging a ghostly blade and disappearing.
Shiv Drinkstep hews bone and sinew, while Cleansing Waters dances between blows, dealing bare-knuckled counterstrikes. Kraz Formalhaut flies to the higher level, looking to take down the archers. One of the magic users flees, but Bess Ashglade and the other cast flames at their enemies. The ooze leaps from the mouth of one of the knights, with Waters barely dodging its eerie probing tendrils.
After her own burst of chain lightning, Sooney-Crow of No Family blows the Horn of Graxus, which awakens the Grey Guardian - it charges directly towards her and into the fray.
The Guardian swings its mighty axe again and again. Sooney takes several blows while the party does its best to protect and rescue her. Waters redirects one of the blows into another of the corrupted knights. Shiv shines the Lantern of Graxus in the Guardian's eyes (really just a lantern) and invokes the Iron God directly.
The Guardian turns its attention to Shiv, who lifts his black blade...
And beheads the Grey Guardian with one clean swing.

The rest is just details. Lord Calder materializes and then vanishes, finally free to go as a vision of Graxus himself dances before the party's eyes.The surviving corrupt knights fall to the ground as the ooze within dissipates. The sounds of battle outside quiet. The Guardian itself turns to smoke, leaving for a moment a humanoid-shaped mass of grey goo, which splashes to the ground and then disappears in a mist. The other grey oozes slip through cracks in the floor unseen and forgotten.
Shiv immediately bends the knee, as does Sooney.
The Brotherhood gathers itself, collects the head of the Grey Guardian, and throws open the doors of the parapets, gazing upon the battlefield below, which smolders from war and dragonfire. Ur-Balat circles in the distance.
Greer the Betrayer declares victory as Shiv lifts the head and the blade.
Greer's claim to the throne is wholly fabricated, but his leadership is not, and the people raise their voices to cheer him and praise his name...
