Cansellarion is willing to offer to send the garrison to the Worldwound if they surrender. In a different war he could take their weapons and their pledge not to rejoin the war, but that's not really an option here; House Thrune will attempt to force them back into service regardless. And he doesn't have the ability to hold them as prisoners. The wizards and officers, if they prefer not to go to the worldwound, can offer their parole, on the condition that they remain outside of Cheliax where it would be easy for them to be forced back into service. Or they can sit in prison cells until the war is over, there are few enough of them that that's feasible. Arvanxi and the rest of the city government will be removed from their positions but not otherwise prosecuted or persecuted.

The same offer is extended to the Hellknights, of course. If their Lictor has other proposed terms he would accept, Cansellarion is willing to hear them.

The Hellknights have orders from their monarch to defend Westcrown and the river. If the offer's not confidential they can ask if she would prefer they surrender so they can serve Cheliax at the Worldwound instead of dying here, but they do not expect it. 

Arvanxi is a practical man. Not even Lawful Evil, he's just Neutral Evil. He's not ideologically Asmodean, he's ideologically in favor of Aberian Arvanxi. He's pretty sure sending Westcrown's garrison to the Worldwound is putting them to death but in a manner that may be elaborate enough the men don't revolt, and he's all in favor. The assurances he'd like are that he will be personally safe from Abrogail Thrune's revenge - he's thinking an overseas vacation, to Sothis - and that his property rights will be respected. He owns quite a lot of the city, see. 

Cansellarion was under the impression that Hellknight orders asserted at least a nominal independence from the Crown. If they consider themselves bound by Abrogail's orders and cannot surrender, then he will unfortunately have to kill a great many of them.

The reclamation would be happy to provide Arvanxi with transportation to Sothis, though they cannot make any promises on behalf of the Pharaoh who is not even, technically, an ally. Cansellarion is also not willing to promise that Arvanxi will be allowed to continue to own all of his real estate, but he will be compensated for any that is seized, at five to seven parts in ten of the assessed value. His slaves will be freed. He'll be compensated for them at one part in ten.

No, Arvanxi's in fact going to play hardball about his compensation. He doesn't think Cansellarion wants to sack his city, and he's sure it'll cost him more than paying out the full assessed value of his real estate. They can tell people it's five parts in ten, if it's the precedent for other Chelish nobility Cansellarion's worried about, and if he gets the other half in gold right now. He'll be gracious about the slaves, he knows that paladins aren't very rational about that sort of thing.

(Most of the Hellknight orders in Cheliax assert varying degrees of independence from the Crown. The Order of the Rack was founded after the Chelish Civil War, runs the censorship apparatus, and is much much less so even under ordinary circumstances, and pledged themselves to the defense of their country when Citadel Dinyar was attacked and Cheliax then invaded.) 

It is the precedent that Cansellarion is worried about, but paladins don't lie so he cannot pay Arvanxi half under the table and pretend he didn't. He will obviously go to some lengths to avoid an assault on Westcrown, but those lengths include trying to remove Arvanxi from his position and do not include paying him a truly massive amount of money in exchange for cooperation, especially when cooperation without compensation already leaves Arvanxi better-off than fighting.

Oh, come on, if the constraint is just that he can't lie that barely limits their options. Does Cansellarion have a friend who might donate some money to the cause of Arvanxi tolerating this patently unfair deal? Happen to want some relics of old Arodenite Westcrown which Arvanxi knows the location of? Want Citadel Rivad betrayed? There are a lot of ways to explain why money exchanged hands while passing any truth spell; as long as it's enough money, he's happy to arrange plenty of justification for him to have it.

Paladins don't engage in elaborate deceptions. If Arvanxi is offering more than just the surrender of the city they can negotiate that separately. If he remains in the city and doesn't surrender, the Glorious Reclamation will take the city, some of his property will be damaged and all of it will be seized without any compensation, and he'll most likely be killed in the fighting. If that happens he'll be safe from Abrogail, though, unless she's particularly mad at him and his trial is prolonged.

The fancy guns really seem like they'll be a lot less useful in house to house fighting; Cansellarion only has them grossly outclassed at range. It's odd that paladins are so stringent about not lying and so determined to steal things, but he'll tolerate the half-compensation for everything else if he gets to keep his manor and property in it.

Cansellarion is not in fact committed to seizing any of his real estate, he just doesn't want to commit to not seizing any of it. Cansellarion is willing to guarantee Arvanxi's manor and any property in the manor continuously from this moment until the city's occupation is complete, excepting property which is also people.

Fine. He's going to need twenty-four hours and possibly to assassinate a couple of people but the city will surrender. Pleasure doing business with you, etcetera etcetera.

Cansellarion will give him twenty-four hours. He'll need the time to take Citadel Rivad anyways. He could do it with his army, but as Arvanxi noted his guns lose a lot of their advantages in close-range fighting. And they don't have siege guns. Are any of his archmages willing to deal with a fortress full of Hellknights for him?

Alfirin can do it, though she'd like to borrow a cleric for an earthquake and the aftermath won't be pretty.

…Anyone else?

Morgethai is willing to provide specific operational support now that Andoran's in the war anyway, but she doesn't have in her back pocket some secret remarkably good way to take a fortress.

Nefreti generously offers to have her location unknowable while Alfirin's doing it.

…They have enough powerful clerics on their side. They can provide the earthquake in the morning.

In the morning, the earthquake shakes the ground beneath the citadel. Most of the superstructure holds; some of the floors give out, some hallways are blocked, some exits sealed. Alfirin boosts her spellpower with prayer beads and a ley line and some animal sacrifice and drops a pair of extended mythic cloudkills that should be strong enough that a pit fiend won't be able to dispel them. The clouds of death spill down the fortress' stairways, like normal cloudkills, pool briefly in the fortress' deeper levels, then rise up the stairs again to hunt down survivors.

The Hellknights of Citadel Rivad have of course prepared for this, but under a few key assumptions: the stationed pit fiends would be able to easily dispel them, or at least to with some difficulty dispel them; the interior stairwells would be unobstructed, so 'run up past the cloudkill before it kills you' would be viable for everyone who wasn't weak enough to die instantly, most people, being hardened veterans, wouldn't be weak enough to die instantly, and once one had run up past the cloudkill it would not be able to come back.

In a way, a cloudkill that breaks all of these assumptions at once is more dangerous than some spell they've never seen before; they think they know how to handle it, and they are dead before they realize otherwise. Some of the people in the fortress get Air Bubble or Delay Poison up in time, but not most of them. Priests give the men strength to overcome the poisonous fumes only to watch them drop dead anyway.

The pit fiends try to dispel the spell. It should really work. It should really work.

After the cloudkills have been at it for twenty minutes (They'll last almost an hour) Alfirin enters, invisible and mind blanked, to check for survivors. She expects to find devils, the cloudkills won't have affected them, and not very much else.

One room of people who got to a scroll of Communal Delay Poison, one room with a wizard who had Life Bubble, a fair number of devils, and a lot of dead bodies.

How many greater devils?

Half a dozen.

Gathered together or spread out?

Spread out; looks like they're gathering the valuables and people worth resurrecting before they teleport out, and trapping the citadel for the incoming invaders.

Then she can try to pick them off in isolation, hexing away their ability to resist and then trying to land a dire prophecy. When the dire prophecies land she follows up with a dominate that they won't resist. Pit fiends first, of course.