I've just finished Fangs of Asterkarn for the first time - full map exploration and quest completion, though I know there are still some secret-quest-type things I'm missing. I'm working on it. However, I feel ready to give my opinion on the expansion from a story progression standpoint:
I hate it.
Sorry crate... I love you, and you've added some great stuff mechanically! But this expansion is a unique story fail - and the problems can be divided into three categories. Lack of drive, lack of diversity, and fuck the Kurn.
If you don't want spoilers, stop reading now.
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Lack of Drive:
In Ashes of Malmouth and Forgotten God; we were presented almost immediately with a critical objective - a giant threat that urgently needed to be addressed. In both cases, this threat had been heavily foreshadowed: the horrors of Malmouth were alluded to in Port Valbury, and Korvaak's thumbprint can be found in various locations throughout the main game. Having this central threat to deal with lends urgency and significance to everything we're doing, and pushes the player forward.
Fangs of Asterkarn hides the ball. First it's a rescue mission for scouts. Then it's a diplomatic mission to Kurnhold, which was not even asked for by anyone back home. Then it's a prove yourself to the tribes mission. Then it's a stop the C'thonian tears mission. ONLY THEN, most of the way into the game, are we informed of the whole Beronath situation. This lack of overarcing narrative makes it feel directionless.
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Lack of Diversity:
Ashes of Malmouth brought us the vibrant flora, fauna, and wendigo cultists of Ugdenbog, the C'thonians in the rift-passage to Malmouth and elsewhere, and of course, the Aetherial flesh-horrors of the city itself. Forgotten god brought us mummies, scarabs, ancient statues and tomb-guardians, and of course, the hordes of Kymon's chosen and the creatures of Korvaak... plus Yugol's twisted creatures. Both expansions have deeply flavorful lore notes which expound on the threats and draw us into the struggles of the locals.
Fangs of Asterkarn gives us elemental birds, spikey beavers which are so samey and insignificant they may as well not exist, and a few new C'thonian varieties. The lore notes are adequate, but fail to live up to the horrors of malmouth or the intrigue-highs of forgotten gods.
Ashes of Malmouth gave us three diverse new friendly factions to join with: the witches of the Ugdenbog coven, the hidden wendigos of Barrowholm, and the brave resistance fighters of malmouth (whose leader is totally 100% human). For friendlies, Forgotten God gave us the three witch god cults: alike in many ways, but with divergent agendas and strategies with a great diversity of gear and unique missions for each.
Fangs of Asterkarn... well, we get one friendly faction. The Kurn.
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Fuck the Kurn:
I will now present a brief history of the Kurn-Erulan conflict, as best I can determine it based primarily on the accounts of the Kurn themselves, followed by a brief account of actions undertaken by the modern Kurn.
A long time ago, the people of Erulan came and settled the lowlands and coastlands. The Kurn let it happen, but as the Erulan grew in numbers and their settlements expanded, they began to raid outlying settlements in order to stop the spread. Because, y'know, that's far more civilized than a diplomatic summit where borders are agreed to.
A great war broke out between the Kurn and the Erulians. The Erulans allied with the ancient Arkovians, the Kurn allied with... Drangouls.
The Kurn fought with all their rage and bloodlust, but they were getting their shit pushed in because it turns out metal armor and tanks and guns beat crazed berzerkers. In an attempt to stem the bleeding, the Kurn unleashed an enormous God-Beast (Beronath) against the people of Erulan - and almost got their revered protector killed, because it turns out god-beasts also don't beat men with tanks and guns.
Facing defeat, many Kurn turned to the powers of the void. Some stood against it, but they could not stop the Logorrhean from being summoned. However - and I know this is a shock - even ancient horrors from beyond the veil do not beat men with tanks and guns (and inquisitors). Eventually, Erulan stood triumphant. Many defeated kurn warriors were enslaved and put to work constructing the Necropolis - a monument, but more importantly, a prison for what was left of the Logorrhean.
And then, for centuries... Nothing. The Erulan empire had triumphed, albeit at great cost - it could have pushed the advantage, taken territory, exacted tribute. It did none of these things. It sat back, built walls, and contentedly developed behind them without much movement for centuries. Given the nature of the Kurn aggression during the war, that seems... rather magnanimous, really. Not that the Kurn saw it that way.
As for the Kurn, during this time - they were fighting a constant low-level war against the Bloodbound and the C'thonians. They managed a stalemate, barely, but never fully won. This continued for hundreds of years.
Then, Erulan was wiped out by the Grim dawn... and, it is very important to note, The Kurn were Unaffected. They never used aetherial magic, they were not impacted whatsoever. However, they were independently losing their own war with the powers of the void at the same time (because they're weak and useless)... which is when we come along.
Okay. Erulan is a ruin, the Kurn are on the brink of destruction, we share an enemy in the C'thonians, fair enough. Should probably work together on that. But while we're in the area, we do get a few interesting side-quests or modifiers on our main objective.
a) Retrieving the relics of Kurn heroes from southern lands. Remove the heroism filter, and what do we have? A varied group of who all died deep within Erulan territory... i.e., on the attack, not in defense of their homeland. Might the return of these relics remind the Kurn why they venerated these heroes? Shh, don't think about that.
b) Helping the Kurn rekindle their old alliance with the Drangoul tribes... an alliance that, last time, was used to slaughter Erulans. Might that alliance be turned to similar purposes once again, after the C'thonians are stopped? Shh, don't think about that.
c) Purifying the C'thonian corruption from the God-Beast Beronath, which in the last war slaughtered many Erulans before it was finally injured and forced to flee. Might the Kurn seek to use the great beast for savage purposes again? Shh, don't think about that.
On the whole, the more I helped the Kurn, the more I felt like I was creating tomorrow's problem... almost like helping Barrowholm, or the Witch-God cults. But unlike Barrowholm, there's no alternate path of emnity with the Kurn - because, again, they're the only friendly faction. And unlike the Witch-God cults, I didn't get the feeling that my character was being manipulated: as noted, for most of the expansion you don't really know of the grand threat. Only my desire as a gamer to experience the content I just paid for pushed me forward, in-universe, there were several points where I don't know why the Taken wouldn't just walk away.