The campaign is possibly salvageable, but it will require significant rewriting. You could start over, or maybe play that whole scenario as a "what might have happened" or if you really want to keep going, immediately go on to Book 2, and have more soldiers, more guards, more everything to really hammer home how badly they blew the plan.
My advice is to start over, and this time don't be so bound to the book. The PC's don't know what's 'supposed' to happen, so long as they feel threatened, challenged and ultimately have the opportunity to succeed, who cares what the book says? Make players pay along the lines of reduced magical treasure or being mocked by their superiors for struggling, but failure or death isn't any fun for anyone.
Have it happen with a better army that was needed elsewhere and the party had no "need to know" and so weren't told about. There's a serious impact to this (demotion, even longer odds in the next assignment, etc.)
My party had the same thing pretty much happen and they they were 1 hit point away from a party wipe. I didn't kill Sakkarrot having read how he plays in down the road but had the bugbears devastated and now the party is on Thorn's bad side. Rather than give them extra levels though I am giving them some menial, worthless tasks to do as punishment while the other knots help get the job started.
For me, it depens very much by how your Pcs have acted.
Thorn had always a reserve plan in mind, the Ice Axe with an army of giants and such.
Also, Thorn may want to conserve a minion like Sakkaroth and revive him in a gruesome manner.
The true question should be: how does thorn punish the Pcs for their immense incompetence?
Does he kill them in order to have them animated as undead? Does he utterly destroy them?
Does he create a 10th knot to dispose of the failures? The choice is yours, but the main concert is everybody fun.
However, if your players are lazy, you should punish their characters inability to face the challanges ahead. How old are your players?
Redo the whole campaign, one living pc would be destroyed by Thorn imo. One living is not a knot. If they are greener then tell them it's a rewind back to leaving thorns villa and they are a different knot, change enough to keep them guessing and rp thorn telling them precisely what wend wrong and what will happen to them if they fail. That is not a beginner AP imo as they can not make many mistakes without the campaign failing.
Good luck, wish ya the best, was a blast for me to gm.
One other thing, they are in a lg land, torture and immediate executions are normally frowned on and the pc's would know that as they are from there.
it was only Day 2
if it was only day 2 when they fired the firework, Sakkarot and his army could not charge since they arent even there.
Sakkarot explained that after the PC departure from his camp, he will need 1 week to regroup and 1 week to travel.Which means, 2 weeks while the PC have to hold and can't expect any help from the army.
Which also mean, no risk for this army and leader to be slaughter.
That can be a solution for your problem.
Actually the army didnt attack, and your survivors PC have 2 weeks to regroup, gather new plan, get new allies ?
In the meantime, the Commander and his people wont be inactive and more search group would look around + the patrol within the castle doubled.
But at least your group is still on track, with a bit more difficulties to punish them for their foolish attempt.
it was only Day 2if it was only day 2 when they fired the firework, Sakkarot and his army could not charge since they arent even there.
Sakkarot explained that after the PC departure from his camp, he will need 1 week to regroup and 1 week to travel.** spoiler omitted **
Which means, 2 weeks while the PC have to hold and can't expect any help from the army.
Which also mean, no risk for this army and leader to be slaughter.That can be a solution for your problem.
Actually the army didnt attack, and your survivors PC have 2 weeks to regroup, gather new plan, get new allies ?
In the meantime, the Commander and his people wont be inactive and more search group would look around + the patrol within the castle doubled.But at least your group is still on track, with a bit more difficulties to punish them for their foolish attempt.
dont forget, they also have the 2 weeks to get a new firework
You should not handwave their mistakes, unless your players are "new" to the game (less than 3 years of experience, i say). Therefore, since they should grew motivations to hate mister T, i suggest you to be merciless. At the same time, you should explore the conseguences of evil and, most importantly, the trend of "rising to power". All evil pcs want MOAR POWA, and you should give to them ... though not painlessly.
Have them pay for their failures in a most gruesome fashion: Tiadora comes by with sakkaroth, alive and furious for the "decoy" trups he had send (truth to be told, he send just a group of incompetent, impatient people guided by an irrational leader (around 35% of their forces); he's more angry for the iron lost than for other things. Ad your pcs just believed to see Sakkaroth die? Maybe ... or maybe there is a darker truth behind this that, if you stil play, can explore in further parts of the story.
Tiadora however is not so kind: she almost slaughter them, then she tortures them until sufferance is the only thing they can hope to know ... and then she stats wispering them how they can redeem themselves, how they can grow beyond ...
Basically, after long torture (one day or two), she will "suggest" in their minds that the only way to elevate themselves is to damn right there a piece of their soul offered to hell (yep, this works also for people already bound to hell like the anti paladin). So she tear a piece of soul from everyone (those who refuse are horribly scarred and shakened instead). This soul piece became an Imp in from of them. And the she "rebounds" such soul piece to the all whole.
This basically makes all your party gain the devilbound template, imp.
such template is useful for: gives them free invisibility, polymorf into 1 animal (each choose a different one), Advances their mental scores (wisdom of hell) and, most important, grants them regeneration (good). This is however lessened by the fact that most of pc enemies will be able to overcome that with ease.
This makes game wise a great twist: pcs now are far stronger and resilient, empowered by hell Storywise, This opens you many possibilities, like a conflict thorn - devils which is intend in the later books. Why tiadora did this? Did really thorn ordered this or she has an hidden agenda?
Now, about the follow up, the orders are changed: PCs now have to SOLO Balentyre by themselves, and after the bloodbath, to Resurrect that slice of sakkaroth army in a gruesome ritual that will see hell's spirits roam in the remaining carcasses of the dead bugbears, animating most of the as Barghests. Only them Sakkaroth shall be called.
So, at the end, you get:
1) players with better characters, and more wise of conseguences.
2) characters more bound than ever at the story and with a new conflict. they hate this servitude? they love it? where are those imp-souls now? what can happen to them?
3) Sakkaroth horde strenghetned by the very forces of hell (like loose 300 bugbear, gain 80 barghest)
Yes, i'm a very gruesome DM. but it is the campaign that inspires me.
Obviosly though, this idea should be presented (vaguely of course) at the players first. if they accept, good. otherwise, their way of the wicked is ended.
The notion that it was only the second DAY and the bugbear army couldnt be there until the second WEEK is the most important here I'd say. It would be incredibly unrealistic if the army did appear so much before time just to get wiped out. Unrealistic and dumb of Sakkarot to launch an attack when he knows himself that hes not ready.
Basically the biggest problem your party has now is that theyre half dead and exposed.
The good side - the fort crew thinks theyre idiots and may even send out hunting groups out of fort to overwhelm and caputure whoever is still uncaught. Simply by brute force and numbers since the knot did not display any more brains than that and Balentyne has a clear upper hand in those areas.
That means they are even less careful about smarter moves and have their backs turned away from where sakkarots army will come.
Bad part again it sounds like your players are not savvy enough to make use of that. You might have them fight off huntingparties though. If the castle crew splits to look for them the separated groups might be an easier fight, though your party still need to filo in the gaps it just gained