We start the chapter with a view of Valkorion on his throne, flanked by his two sons. Adult Vaylin appears and tells him that he should have left her on Nathema. He responds that Zakuul is where she belongs and hands her a lightsaber. There is a brief moment where it seems like she might strike at her father in anger, but then she kneels to him instead.
Your character wakes, revealing that this was something you dreamt. Valkorion's ghost comments that this is a sign of your increasing connection, since what you saw was one of his memories. You ask about Nathema and he explains that it's a "blighted, forgotten world where the Force has been corrupted" (if you read the Revan novel, you know all about that already) and that he sent Vaylin to be taught and conditioned there. He also suggests that the timing of this dream is a warning that Vaylin is seeking to return to Nathema to break her conditioning.
When you walk in on Theron and Lana speculating where Vaylin's disappeared to shortly afterwards, you suggest Nathema as an option. Theron locates it and realises that Vaylin had communications with someone there during the party. If Arcann is one of your followers now, he expresses fear of returning there and that being exposed to its taint might undo everything he's achieved since Voss, which is why he wants to stay behind.
Either way, you travel to Nathema by shuttle, only accompanied by Lana and Theron. Lana immediately starts having headaches upon arrival, as the Force is kind of "hollow" on the planet, and it feels like the void wants to devour her. Valkorion's ghost appears to flicker in and out of existence as well. We see Vaylin landing at a remote facility, speaking to an Anomid scientist and insisting on entering alone, without her guards. You land nearby to retrace her steps with Lana while Theron stays with the shuttle.
Meanwhile, the player sees Vaylin walk through a sort of indoor garden where she's assaulted by a number of monstrous beasts that bear some resemblance to Acklays. She is momentarily frightened when she realises that she can't use her Force powers here to swat them aside, but then she simply pulls out her lightsaber to cut them down manually. The Anomid scientist we saw her talking to in chapter six, Jarak, expresses distaste for her lack of control and the killing of his "pets". However, he also says that he made a breakthrough in terms of breaking Vaylin's conditioning and offers to show her to the main lab.
You and Lana fight your way into the "sanitarium" in her wake and find traces of what Vaylin must have gone through in her childhood, from her literal cell to holojournals detailing the gruelling treatments she underwent at the hands of Jarak, who was given free reign to do whatever he wanted to her short of getting her killed.
The scene returns to Jarak and Vaylin, where the scientist confesses that none of his lab rats survived his new treatment so far, but he is sure that Vaylin will be fine due to her strength. We see her getting engulfed by green lights and scream in pain as she unleashes a destructive wave of power. Even you and Lana can feel it and push ahead to stop the process. You run straight into Jarak, who's trying to flee the scene. He worries that Vaylin's power will destroy the facility and that there isn't enough time to escape. He offers to lead you to a safe place however, provided that you help protect him on the way.
Your safe haven turns out to be a vault that is shielded against Force powers due to Valkorion having used it to store particularly powerful and dangerous artifacts. Soon after you enter the vault, we see Vaylin freeing herself from the painful grip she in in. If Senya is still alive, she will wake from her coma during the height of Vaylin's pain, which makes for a nice throwback to the Betrayed trailer.
The burst of power has overloaded the circuits that open the vault door, forcing you to look for a way to restore backup power. A mysterious red holocron catches your eye, and when you touch it, it reveals the spirit of a Pureblood Sith Lord, who seems to have expected the Emperor - to whom he refers as "Tenebrae" - to come and torment him again. He reveals himself as Lord Dramath, the Emperor's father and former ruler of the planet - until his son murdered him and turned the planet into a wasteland by performing a dark ritual, similar to what he did on Ziost.
Dramath begs you to release him from imprisonment and end his existence, while promising that his holocron will serve you as a tool to destroy Valkorion. You can grant his wish or keep him trapped inside the holocron to make use of him later.
Your rummaging around has awoken some monstrous vault guardians that kill Jarak (if you haven't already offed him yourself). Once you defeat them, you can escape the vault. Vaylin is shown to already be back on her flagship, and she enjoys demonstrating her newly unfettered powers by killing most of her own guards without even breaking a sweat. She also channels her hatred into destroying the sanitarium, forcing you to make a run for it to make it out alive, but Theron manages to pick you up before things fall apart. Once off Nathema, Valkorion reappears and confirms that Vaylin has broken her bonds and is now more dangerous than ever. He also seems worried that you might have found something dangerous in the vault.
The chapter ends with Bey'wan Aygo calling you on the holo: You are needed on Odessen urgently as it's under attack.
I thought that "Into The Void" was another interesting chapter. Nathema is compared to Ziost, and similar to that planet there is less of a focus on combat and more on letting the sheer deadness of the environment weigh you down. While there are more enemies to fight on Nathema than on Ziost, there are still fewer of them than in most chapters, and Bioware definitely lets the heaviness of the place sink in.
However, the focus is less on Nathema's own dark past and more on everything Vaylin went through. Even if you never saw the trailer for KotET, this is a turning point where you get to feel some sympathy for Vaylin upon seeing her prison and understanding what she must have gone through. While the story does point this out, the environment invites you to take a closer look and draw your own conclusions. For example I found it quite heart-breaking to find the "cell" that Vaylin clearly was confined to, containing nothing but a small cot, a desk and a chair. On the desk you can also spot what looks like the little toy she crafted in the trailer... though that would be a continuity error as the trailer also shows it lying in a puddle on Ord Mantell later... but who knows, she might well have made another one. There is a corner with beat up skytrooper dummies that are arranged in a similar fashion as the "knight pinata" she attacked in the trailer - was this the one place where she was allowed to try and exercise her powers sometimes?
You also wonder about the lives of the other people inhabiting the facility - while searching for a key card, you rifle through some books, and on flipping one of them open, I noticed that a postcard from Alderaan had been used as a bookmark - an odd splash of colour in an otherwise very grey environment. Personally I also couldn't help but wonder why the place was such a mess though, with rubbish and random papers on the floor everywhere. There were people living there after all, did they never bother to do even the smallest amount of tidying?
After seeing all of that, you can't help but feel for Vaylin a little when she's trapped in Jarak's machine and in pain. After she breaks free, there is some great work with facial expressions done in the cut scenes, as she slowly goes from a slightly confused "I'm alive?" sort of expression to a sardonic victory grin.
Meanwhile, your character is in a bit of an odd place because you keep chasing Vaylin around but don't really achieve anything on that front. The only interesting choice you get to make is whether to free or keep Lord Dramath, and the significance of that doesn't really become apparent until a bit later.
That said, just seeing Nathema realised in game and talking to Dramath feels great if you've read the Revan novel. So while this chapter is heavy on exposition and explaining Vaylin's (and to some extent Valkorion's) backstory, with a limited amount of action for your character, I've enjoyed it every time because it's just so atmospheric, achieving a very different feel from any other chapters of KotFE or KotET.