After you've found all three Sisters - whether they were already dead when you found them, you killed them afterwards, or you shamed them instead of sending them back to camp (or a combination thereof) - return to the Kellid Barbarian Camp. If you were expecting a hero's welcome, however, you're sure to be sorely disappointed. Be sure to spellbuff as much as possible before moving anywhere upon entering the area.
When you're ready, approach the center of the camp to witness a scene, where the Sisters are proving themselves worthy of the contempt their fellows heap upon them by accusing you of wrong-doing. The outcome of this encounter depends on two factors; how you respond when you get the chance, and whether the barbarians are already biased against you enough to do the Sister's bidding.
In the latter case, their disposition towards you was determined during your first visit to their camp, during the Illustrated Book Event. You had two options to gain favor and two options to earn disfavor, and so long as you didn't didn't pick both negative options - drinking the wine instead of pouring it out and arguing for the Sister to eat with the rest of you - you'll be fine. Or at least, you'll have a chance to reason with Dugath. If their opinion of you is negative, however, Dugath will command his barbarians to attack.
Assuming a fight against the entire barbarian host isn't inevitable, you have some options. First, you can attempt a [Diplomacy 25] check to talk some of the barbarians out of a fight. You'll also automatically attempt a [Lore (Religion) 25] check which, if successful will give you a corresponding option that avoids involving Dugath and a few others. Finally, if your protagonist is [Neutral] you can pick an alignment-specific option to mitigate the upcoming fight.
Some level of conflict is unavoidable as the Sisters and their loyalists will attack you regardless. The fight is, however, much easier without Dugath and his minions joining in although the strategy for this battle is the same either way. There's a lot of barbarians, and you start out in a miserable position so retreating across the bridge to the south (a feat much less painful with Haste) is a mandatory first step, although even then you'll be hard-pressed to keep the barbarians from surrounding you unless you have a warrior-heavy party or summon some allies. Assuming you can break free to the south and form a defensive line, use debilitating spells like Confusion, Web and Slow to hinder your enemy, while putting up buffs of your own. The average barbarian will die readily enough, but the Sisters are more resilient nuisances, having high Armor Class and Damage Reduction. They're hard-to-remove nuisances, and frankly, the more you killed out in the wild, the better off you'll be.
Should you emerge victorious, talk to Dugath, who will tell you where the Sisters were searching - the Valley of the Dead. If you fought him, he hopes you find your demise there, but doesn't help matters by forfeiting some high quality gear. If he didn't fight you, he's a bit more diplomatic about revealing the location of the Valley of the Dead. You'll also get an option to attack him, so if you want his gear without fight all the barbarians at once... Either way, if you kill Dugath be sure to loot him for a Breastplate +2, a Cloak of Resistance +1, a Belt of Physical Perfection +4 and Endless War a Greatsword +5 with the "Vicious" property, which deals 2d6 damage to the opponent and 1d6 damage to the wielder.
However you resolved matters, pick up the Cyclops Incense Burner from the Sister. Another fey creature. Seems you can't get a reprieve from their machinations.