Time for another detailed and spoiler-laden discussion of a Knights of the Eternal Throne chapter. We're up to chapter three! Though if you missed it...
Chapter two ended with the player seeing Koth inadvertently inviting Vaylin and Scorpio onto the Gravestone because he stepped right into a trap of theirs. Chapter three starts with the Alliance receiving a distress call from Koth, asking for help. You decide to go and help him even if he stole the Gravestone from you at the end of Fallen Empire... because obviously you don't want Vaylin of all people to have it!
When you arrive, Vaylin and Scorpio are sitting on the Gravestone's bridge, puzzling over why the ship remains shut down and refuses to let them reactivate it. The Alliance fleet engages the nearby Eternal Fleet ships while you and Lana board the Gravestone via a boarding pod.
You land in the power generation room, stuck behind a locked door and having to play a little mini-game that involves remotely manipulating a mouse droid outside to open up the doors. On your way to the bridge you take out a couple of skytrooper patrols until you suddenly run into Koth. If he stole the Gravestone from you, he makes an awkward peace offering, which you can accept or reject, but for the moment you need to work together anyway, while Lana tries to create a distraction. Koth explains that he was suspicious of Scorpio from the beginning and installed additional security measures to prevent her from taking over the ship, which is why it's floating dead in space at the moment. However, on top of this he fitted the ship with a so-called quantum bomb, which will cause it to go boom big time if Scorpio actually manages to break the code and takes control, meaning your first priority is to disarm said bomb. On the bridge, Vaylin and Scorpio can be seen turning to torturing Koth's crew to learn more about the encryption he used.
When you make it to the bridge yourself in order to take the last step to disable the fail-safe however, nobody but the crew is there. Koth is confused to find his security broken, yet no actions taken that would have triggered the bomb. After he tells his crew to get to the escape pods, Scorpio calls on the holo to say that she manipulated Vaylin into leaving the bridge so you would have time to disarm the bomb, as she doesn't want to see the Gravestone destroyed either. Now Vaylin has cornered Lana at the bomb's actual location however, and two powerful Force users duking it out right there is bound to get dangerous.
You and Koth rush to the Dark Sanctuary, where you find Vaylin pounding Lana into the ground. You interfere and fight her, but she is powerful. Suddenly Valkorion performs his time-stop trick again and steps between you. Vaylin snarls at him, expressing her hatred of the fact that he caged her as a child (which I think is the first time we hear about the events of the KotET trailer in character), but he counters by uttering the phrase "Kneel before the dragon of Zakuul", which has a strange effect on Vaylin, shutting down her ability to fight. She flees the scene, but the fight did set off the bomb's timer. You rush around the nearby rooms to bring down the shield around it, so Koth can disarm it. There is a bonus mission to do this within three minutes, but I have no idea what happens if you take longer... I guess the game just pretends that you still made it in time, you just don't get the bonus reward?
Valkorion explains that Vaylin has been conditioned to respond to the phrase he used and that it renders her unable to use her powers. Meanwhile, Theron arrives in a shuttle with reinforcements and informs you that the Eternal Fleet has jumped into hyperspace. Koth is pleased that the situation has seemingly been resolved to everyone's satisfaction - though if he stole the Gravestone from you and you refused his peace offer, you can punish or even kill him for it now.
Vaylin is shown limping onto the bridge and tries to attack Scorpio for playing her, too, but under the influence of the conditioning she can't bring her full powers to bear. Meanwhile Scorpio announces that the technology that created her, the Eternal Fleet and the Gravestone all come from the same origin, and that she's about to take the Gravestone elsewhere.... "home" as she calls it shortly afterwards. The ship jumps into hyperspace with you and your crew still on board, and you are unable to do anything about it.
Chapter three is probably the shortest of all the KotET chapters, and mainly seems to serve two purposes: to resolve "the Koth situation" and to reveal Vaylin's secret weakness.
Initially I found it kind of odd that Vaylin of all people would go and set a trap for Koth herself. Doesn't she have an army of knights that can do that sort of thing for her? It's a well-worn storytelling trope that the ruler/boss likes to lead the charge themselves even if it doesn't necessarily make sense from a strategic point of view, but still... yet as I thought about it some more, I realised that it actually does make a lot of sense for Vaylin of all people. She appears to have been the executive arm of the law for the entirety of Arcann's reign, and maybe even before that, and she doesn't really take well to just sitting on the throne and letting others get things done. This makes for an interesting contrast with Arcann in particular, whose habit of perpetually sitting on the throne and brooding instead of actually doing anything for most of KotFE was pretty legendary and became the butt of many jokes. The way you and Vaylin chase each other in circles around the Gravestone while continuously missing each other does get a little silly though. There aren't that many different corridors to the bridge!
Valkorion interfering directly and revealing Vaylin's conditioning is an interesting twist. It also gives our characters an opportunity to learn in game what we as players already knew from the trailer: that Valkorion had Vaylin locked up as a child. (Previous mentions of him restraining her powers at a young age had been very vague.) Seeing her frightened expression when her powers desert her makes her at least a little bit sympathetic, despite of her cruelty and raving insanity.
As far as the Koth situation went, I had mixed feelings. People have been clamouring for an opportunity to kill him since the moment he betrayed you at the end of KotFE chapter ten (if you made certain choices), not to menion him running off with the Gravestone afterwards. If that happened to you, getting your revenge does feel pretty satisfying. You get to kind of "betray him back" by coming to his aid at first and then killing him once the worst of the danger has passed, and seeing Lana's reaction in particular is quite heart-wrenching: She is visibly pained by seeing him die, but still stands by stoically as you execute him, knowing that he earned it.
As someone who leans more towards the light side and who had more characters with friendly relations with Koth though... it's all a little disappointing. Like Koth's brief appearance in chapter one, the events of chapter three kind of feel like they were written specifically with Koth the Betrayer in mind. If he's a loyal member of your Alliance, why is he off chasing Eternal Fleet ships in the middle of nowhere? Why is this former soldier that came off as clever and resourceful during the first nine chapters of KotFE acting like such a buffoon now? Not to mention that from my point of view, I would have been more interested in how a loyal Koth would have come to reconcile his adoration for his Eternal Emperor with the fact that I was the one who killed said Emperor in KotFE chapter one and that I continued to fight against his unliving spirit until the end. But I guess that would have required more complexity than they had time for.
Oh, I mustn't forget: I did like the inclusion of yet another little puzzle segment, this time with the mouse droid, though I wish you didn't have to restart from scratch after every step and could instead do the whole segment in one go (if you are skilled enough).