I am finding that after Assault of Stone the AP devolves into a different theme than the first 3 volumes, becomes more traditional adventuring sort of stuff, and most importantly, is just long with essentially not much happening except explaining some back story of why everything is happeningn, but not really (more on it later). For us Volume 4 is getting cut almost completely, and 5 will get beefed up and 6 probably shortened.
Anyway I thought I'd leave this short suggestion of how to cut the Siege of Get to a Dwarf who tells you where to go next cause he uses Scry. Stone. Something. (Yes I've got issues with the terrible overarching plot of this AP as much as I enjoy the individual modules. Onyx deus ex macguffins.)
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BEFORE THE SIEGE OF LONGSHADOW
In one of the sabotaging missions or in the hollow hills the party finds a Dwarf (or more) that are part of a scouting mission from Kraggodan.
Why are they scouting? They either do it often. Or they found that someone meddled with their treasury (something the PCs find out in module 4) so they follow the breadcrums in reverse, but end up captured by the Legion and put to work.
This Dwarf, a cousin of Karbutin so on good terms with him, says he's seen the devastation of Nirmathas, the urgency of helping out and also that the siege of Kraggodan was orchestrated by Azaersi (as they find Elacnida's and Fake Zanathura 's information before being captured). He swears on his dwarven beard that he will convince the dwarfs to sue for peace with Molthune and offer help.
Alternative if too close to the Siege of Longshadow timeline, have him use Send, or Teleport.
DURING OR AFTER THE SIEGE OF LONGSHADOW
A group of dwarf warriors arrive at Longshadow. Either marched from Kraggodan through one of the myriad holes to the longroad in the hollow hills, or teleported. They arrive before, during or after the siege. I would have it during the siege, cinematically stopping a charge of morlocks with their dwarven staltwarness, cementing themselves as allies of the PCs.
Karbutin is amongst them, he has been sent as an emergency delegate; Molthune and Kraggodan are discussing peace, and Karbutin has the info to tell them exactly what he tells them at the end of Volume 4 but wihout the PCs going to one of the most dangerous realms known on just a hunch (underdark...).
Karbutin uses Scry and fills them in the dwarven history.
WHAT ABOUT ELACNIDA?
If the story can proceed without a scene, or an explanation, then it is not serving to the purpose of the story and should not be there. This is common in story building. The Elacnida story whilst cool, does very little to explain the story or help it along. It's my opinion of course.
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THE FANGWOOD, THE SARDONYX SHARD, AND THAT DUMBASS TAURGRETH (but not many changes here)
As written, Taurgreth is a smart dude that does something extremely foolish and possibly betrays the master he is extremely loyal to. He takes the Sardonyx Shard when he is sent to the Accressiel court (and doesnt take an Onyx Shard so he cant even use it, check this thread https://paizo.com/threads/rzs2v15h?The-Onyx-Key).
Furthermore, we get introduced to a new villain that is potentially more dangerous than the Ironfang Invasion that isnt even allied with them. It starts to get silly, and we're not even at Camelot yet.
Here's my suggestion anyway: Azaersi sends a delegation to the Accressiel Court as before, but with a Sardonyx Shard and an Onyx Shard -on purpose- so her agents can return fast or they can send troops back and forth should negotiations prove successful. Or could go personally at first, whatever to make it more official.
Unfortunately, much as in the module, Arlantia has other plans, yet she is not a dumb tree as written, so she intends to play Azaersi and let the Legion break itself and Nirmathas/Molthune/Kraggodan before unleashing the blessings of the V plague. She infects the hobgoblins which will bear and spread the spores as they fight far away. She gets into Taurgreth's mind and extracts the secrets of the Shards.
The PCs arrive and do their thang.
Or Arlantia could be full-on with Azaersi and figures she'll cross the river when she gets to it. Whatever. So long as we clarify the Shard-Taurgreth's behaviour.
Module 5 is fantastic, it almost deserves its own standalone adventure. It is however very disconnected from the theme of the campaign, but hopefully this makes it a little more organic.
Hope you find this useful somehow. This AP once the Onyx Key gets properly involved starts to get silly and seems to be written in a disconnected manner where I feel the direction of volumes 4-5-6 was not set in stone until late and they shoehorned a bad artifact retroactively to explain some rather dubious and lazily written deus ex machina that happens off screen (without Taurgreth's foolishness there is no definitive solution to this AP).
But it's mostly about skipping volume 4 (as cool as the underdark is, we just wouldnt finish this campaign before feeling the railroads get mind bogglingly boring).
Happy hunting, Rangers
I'm a little surprised that if you felt like the Onyx Key wasn't adequately worked into the story and book 5 isn't consistent with the tone of the other books that you'd opt to remove book 4. I'd have thought book 5 would be the obvious candidate to remove, as all you really need to do is get the PCs the Sardonyx Shard.
I might be biased because I'm currently running book 4 and having a blast with it.
Hey Captain Morgan, been enjoying your stories and posts here.
I agree book 5 is probably the better candidate to extricate, but it fits my group better to deal with the fey as we can resolve 2 character backgrounds. Furthermore, I love fey stories and I'm just better and more enthusiastic running them than dwarven ones, so it is purely a personal choice. Since we are doing this I wanted to leave it somewhere in case someone finds it useful.
Both volumes are very weak on the over arching plot line. In 4 you get some background info that doesnt affect a single choice you will make and then get pointed in the direction of the actual artifact. On 5 you stumble accross a huge other problem that deviates from the main storyline and you find the artifact. The adventures themselves, individually, are excellent.
On the tone of the adventure:
As described in the preface of the volumes, each deals with a theme. Book 1 run for cover, Book 2 survival, Book 3 guerilla, Book 4 reorganising, siege, fighting back.
Then Books 4 and 5 go into an investigation that could take anything from weeks or more where you remove the players from the central tension of the adventure, and instead happens in the sidelines. It would be fine for a short spell, but not for 2 books. Then book 6 returns to the natural flow yet there's the big assumption that Nirmathas isnt lost without the party actually dealing with the invasion for 2 whole books.
On the Onyx Shard:
The Onyx Shard works as a premise for the whole invasion being possible, but it has the quality of a over complicated and shoehorned artifact. Just not a fan, it is unusual that a modern story revolving an artifact provides a good narrative. In the case of the Shard it falls into this problem head on. It is overly complicated and requires a lot of explanation, it is easy to misinterpret (even the actual authors did not understand it well), and to me feels like it is molded as it goes to deus ex machina the story into a solvable puzzle. Unelegant.
Simplicity goes a long way when dealing with artifacts (in Jason and the Argonauts the golden fleece doesnt do much; in Lotr the ring's biggest thing is that people want it; Excalibur is a sword that if you have it people will call you King). Otherwise you end up in convoluted mechanics that unravel the closer you look at them.
I realise I am picky, and I know full well the story is a framework to write on and tailor the experience. However the more one has to change it and the less organic it feels the more prep work has to be done. Also different groups will value different aspects of an adventure of course.
I can see where you're coming from, even if I don't share your opinion. I think Ironfang Invasion is trying to both be a classic dungeon crawling campaign and a Scrappy war story at the same time. For me, it does so pretty successfully-- with the notable exception of how little the Militia system intersects with the game's script.
But when a story tries to have its cake and eat it too, things can get strained at the seams. Still, I like the Onyx Key as I think it does what it is set out to do well-- elevates a fairly conventional army into something which can tear through nations.
For me, I intend to entirely cut book 5 honestly. Going to hang out in magic faerie land for a book does nothing for me thematically with this AP (and how I've changed it too). Using shame, and bringing the secret to light about the lost Onyx Key from their own guarded chambers is what will spur the dwarves to act outside their mountain.
But I think it's interesting on how you want to cut the dwarves out though!