Beginning to run out of ways to say "Spoilers!", thank goodness there's only one of these left!
As with the Sith Inquisitor, you arrive on Korriban, ready to take the Sith Trials. One major difference; rather than you being a slave, you are a specially-hand-picked Acolyte. Indeed, as revealed later, you are actually picked too early because of your promise. You are introduced to Overseer Tremel who informs you that you are being prepared for a Sith Lord named Darth Baras, and that you have competition in the form of Vemrin. Your goal is therefore to make yourself seem a worthy apprentice to present to Baras and avoid being killed by Vemrin - although you actually get to talk with him for a couple of scenes.
Despite being a Sith, you are told in a very interesting class mission to be aware of other paths; you have to decide on the fates of three prisoners. You can either kill the first in cold blood, ignore him, or kill him in an honourable duel. You can either recruit the second, ignore her, or kill her in cold blood. You can either keep torturing the third, ignore him, or have him freed. Tremel will commend you for performing the first interactions in all three cases, even though the second is actually a Light Side option.
Baras eventually comes to Korriban after you slay an ancient beast in the Tomb of Marka Ragnos, and he instructs you that you have been poorly educated. You don't even know the Sith Code! You therefore are sent to kill Tremel for his incompetence, although as you only have to bring his hand back, you can let him run instead. Baras will be none the wiser. He sends you to reclaim an ancient Lightsaber from the Tomb of Naga Sadow, using the intelligence of a captured treasure-hunting Twi'lek called Vette to help you enter the tomb. You encounter Vemrin and kill him, reclaim the Lightsaber, and return to Darth Baras, who officially proclaims you his Apprentice.
When you reach Dromund Kaas, you are informed that a captured SIS Operative is being delivered via Carbonite to him. After defending the shipments from two gangs whose leaders, whilst mentioned, never actually show up, Baras attempts to squeeze what he knows from him, infamously leading to a scene where he bellows about his inability to "break him". The Sith Warrior, by the way, has the propensity to make humorous quips in situations like this, and they are all hilarious ("Problem, Master?"or "Nice Lungs" or "I feel your anger"). After berating you for choosing whatever quip you desired, he sends you to the Dark Temple, wherein lies a special tool used by the Emperor to learn his enemies' secrets; The Ravager. Through this tool, Baras learns that a Jedi Master,
With this threat established, you're off to neutralise Baras's spies and discover the whereabouts of Karr and his Padawan.
Balmorra and Nar Shaddaa see you locating two of your master's spies and killing them. The chap on Balmorra is actually posing as a Republic Commander. However, the Republic know something is up, as a Jedi is investigating him and his son has even been imprisoned despite him being a proper Republic citizen. The mission where you have to kill him is also the first of a couple of toilet-humour jokes ("He's going to need a new pair of pants"). I love how such a serious and powerful character such as the Sith Warrior can be so snarky. It is just so ridiculous but so hilarious. Anyway, the "Republic Commander" is aware of your purpose and faces his death gallantly as a true Soldier should. You nullify the Jedi's attempts to contact Nomen Karr with the aid of an Imperial Lieutenant by the name of Quinn, and kill her as well.
By contrast, the spy on Nar Shaddaa is a coward. He runs and finds protection with a Sith Lord by the name of Rathari, who is one of Baras's many rivals. You have to stop his own initiatives and weaken up in order to get close to him. One such initiative is preventing a deal with two Hutts, which leads to perhaps the best scene ever for Dark-Siders; after knocking Rathari's apprentice out, you promptly kill the Hutts in a powerful flourishing swing aided by pure momentum. I'd actually recommend seeing this particular cutscene multiple times because it is so well-done. Anyway. You encounter Rathari and, after a duel, he kills the spy who sought refuge with him and pledges himself to your service. You can either take him up on it or just kill him outright.
After this, you are informed that a Republic ship is tracking you, so you make way for it. Here, you discover that Nomen Karr is aware of your presence and you share a conversation with him via holo. You, in turn, reveal that you and Baras know about his apprentice.
Baras has learned that this Padawan was trained on Tatooine, so you head there to find out what she learned and who trained her. You discover that she managed to pacify a great Sand Demon, so you in turn try to pacify it or, in turn, kill it. Either way, you're redirected by the Sand People to a cavern where you can literally reflect on your person; a Light Side manifestation of yourself emerges, and tries to convince you to abandon the Dark path. You can either accept or kill it. Either way, your next objective and the location of the Jedi Master Yonlach, who trained this Padawan, is revealed. This leads to the second toilet humour joke; your guide is absolutely petrified of your planned location, as nobody has ever knowingly returned. You can tell her to, quite literally, "go wet (her) pants somewhere else". Okay, snarky is one thing, but continually making use of this style humour? You find Yonlach and another Jedi and, after they paralyse your companion, you have to defeat both of them. The other Jedi reveals the Padawan's name: Jaesa Willsaam, and that she was from Alderaan.
Your task on Alderaan is to locate Jaesa's family and kill them so that she would feel pained and want to seek you out. You are helped by a very unwilling Duke Kendoh in this matter, who wants to see his own goals furthered more than he does your own. For example, he directs you to a Lady Renata Alde on the premise that Willsaam was her handmaiden; she never knew her, but knows that an Organan General, Giselle, had a handmaiden who fits Jaesa's description. Kendoh directed you to her because he wants to have his way with her. Slimy git. You enter Giselle's fortified bunker in perhaps the only known instance of a Lightsaber cutting a door open in SWTOR's cutscenes and she promises to give up the location of Jaesa's parents if you help her defend her front. You can either accept or kill her lover and force it out of her before killing her as well. You locate the parents and either kill them or promise them a life of luxury in the Empire; either way, Jaesa would know and be mortified. Baras contacts you and informs you that Kendoh had been making you out to be troublesome and so you go and deal with him.
Back on your ship, Jaesa herself contacts you and offers to meet on a ship to the effect that your quest can end and she can duel you in retaliation for what you have done. This is in turn a trap, as Nomen Karr learned of her plan and told two Jedi Knights to go in her place. You soundly thrash both of them (although light side choices will result in one abandoning the other), and Baras tells you that Karr has challenged him to a duel on Hutta. You are sent in his stead.
Karr gives a hell of a fight. He doesn't back down at all, and actually turns to the Dark Side in order to try to defeat you. When Jaesa, who of course felt her master's pain, arrives, she can't believe what he has become. She will then analyse you; based on your alignment you either appear a true agent of evil or a force of light. Either way, Jaesa fights you for what you did to Karr, and afterwards you can convince her to either turn to the Dark Side, even having her kill Karr, or to remain on the side of Light. If you care at all for Achievements, please not that this does mean Jaesa counts as two companions for the 10k affection.
Having a companion be either Dark or Light is very fascinating, and it's a shame that we don't see this with Ashara in the Inquisitor stories or even with any Sith companions in the Jedi storylines. That said, seeing the two separate achievements for Jaesa probably also means that they'd have done two for all the other flexible companions, and I don't see many people being appreciative of that...
The chapter ends with your mission a success, and you are proclaimed a Lord.
The second chapter begins with you being dispatched to Nar Shaddaa to disrupt a meeting between Imperial traitors and Republic officials. The Imperials reveal that a Jedi named Xerender has arrived on Hoth before you burst in and proceed to kill the Republic men and the Imperials, unless you allow them to come back without recompense. You are then introduced to Darth Vengean, a Dark Council Member and Darth Baras's superior. Vengean is striving for war and is enacting Plan Zero, which would subtly remove important figures from the Republic's arsenal and eventually lead to war. Returning to your ship, Baras informs you that he has located four generals on Taris but two individuals - Admiral Monk and Jedi Knight Xerender - evade him. Even though you overheard Xerender's location you infuriatingly cannot relay it to Baras.
Taris sees you killing each general one by one, each one sure that you are after them for their special military project, and each time being shocked to discover that you just don't care and are just after them for another reason. One general even manages to trick you into the Super-Reactor which that Kaleesh tried to destroy in the Consular storyline, and actually ends up destroying it in an attempt to kill you. For some reason, despite having the big build-up of causing planet-wide damage, the damage caused by this particular explosion is minuscule. That Kaleesh truly was insane. You eventually kill them all and are praised by Baras.
The Warrior gets the final interesting Quesh interlude. Admiral Monk had attacked one of Darth Vengean's ships and had gone down to Quesh. When you try to rescue the crew, the Moff in command of the bridge accuses you of being part of Monk's scheme and tries to kill you. He obviously does not succeed. When you catch Monk, he reveals that he and his men are, in fact, working for Baras. For some reason, his true allegiance was kept hidden from you, and again infuriatingly, you can't just say "you didn't know" as part of your options after hearing this. It's no doubt perplexing why Baras would guide you to kill his own man without telling you who he really was, though.
On Hoth, you are of course after Xerender. It transpires that he is trying to free a Jedi Master by the name of Wyelette, who reveals that in his solitude on Hoth he has communed with the Force and has no interest in the Galaxy or the War. You can either honour his request to leave or just kill him. You also have to avoid interfering with the hunt for vengeance by a Talz named Broonmark, and that's really not easy considering that your mission objectives directly lead to areas where he then pops up again.
With these seven powerful individuals dead, Baras reveals the truth behind Plan Zero: it was of his design, and the Dark Council does not appreciate the subterfuge performed. Because it was done under the name of Vengean, however, Baras claims that the Council is calling for his head and that killing him would cement his position and make him a Dark Council member.
He guides you to freeing an apprentice of his, Lord Draahg, who he placed in Vengean's service. You tear through his private sanctum and confront Vengean together. He informs you that your talents are wasted on Baras and momentarily considers the face of Baras if he went against him with you at his side. Nevertheless, you have to fight and kill him, and Baras assumes his place on the Dark Council. Your position, therefore, is even stronger as the Apprentice and ally of a member of the Dark Council.
The third chapter sees you initially sent back to Quesh. Draahg is in need of your help in dismantling a Republic bomb effort, but once this is done, Draahg proceeds to try to kill you. He knocks you unconscious and leaves you for dead. When you awaken, you are greeted by the sight of two Pureblood Sith, who identify themselves as Servant One and Servant Two of the Emperor's Hand. They name you as the new Emperor's Wrath, and inform you that Baras, upon ascending to the Dark Council, has claimed himself to be the Emperor's Voice, essentially making a claim to rule the Empire. With the true Voice locked away, his claim looks almost indisputable, so your task now is to stop Baras and kill him.
Baras's mission on Belsavis sees him sending a Sith to free his long-imprisoned sister, Darth Ekkage. You ally yourself with a Jedi Master, Timmns, and together you defeat her assassins and eventually take her down herself, either leading to her re-imprisonment or her death. Either way, Baras's plan is foiled. You then have to go back to Hoth to allow troops redirected by Baras to go to Corellia - sounds familiar... - to support his strongest and most outspoken rival, Darth Vowrawn. Upon re-entering your ship's airlock, you find Draahg, who has knocked out all your companions - save whoever you brought with you - and you duel him. He apparently "can't be killed", but you get round this by knocking him from the walkway and into a blazing inferno which, ironically, he started.
Voss is a very interesting mission. It transpires that the real Emperor's Voice is a Voss Mystic held in imprisonment by the Dark Side Entity Sel-Makor, who is in league with Baras. At least now we know why the Emperor's plan didn't involve killing everyone directly. That said, this actually provides some confusion. Voss was discovered five or six years before the game's starting point, and the Emperor's Voice has been a Voss for at least this long. Who, then, do we meet in the Space Station as a Jedi Knight? Is it the Emperor's main body acting without a will? Is it the Voice, somehow able to return to his body despite being imprisoned by Sel-Makor at this point? It's genuinely perplexing, and this story doesn't help matters: Sel-Makor's imprisoning the Voice means that his spirit cannot travel back to his main body, else he would foil Baras's plan, and yet we clearly hear the Emperor's voice emanating from the body in the Knight's storyline - all Chapters take place in the same year as each other across three years, by the way. I digress. Because his spirit can't free itself, you actually have to kill the Voice's host body in order to free it and allow it to return to its actual body.
Before you can move on to Corellia, Quinn tells you that he discovered a listening post which has proved detrimental to the Empire and so must be destroyed. When you board the station and reach the main chamber, however, he reveals that not everything has been as it seems. Darth Baras has been his true master all along, and he has bade him lure you to the station so that you can be killed. After insulting whichever companion you have out (Vette and Pierce are particularly hilarious), he stuns them and you must face him and two Droids alone. After defeating them, you can choose to forgive him or choke him to near-death. Having a companion betray you has never happened in the game before this; whilst you have to fight various companions, this is before they join you. With Quinn, though, he's of course fully-established and ordering your crew around by this point. A lot of people hate Quinn because of this - if they didn't already!
With this done, you move on to Corellia to save Vowrawn and prevent Baras from manipulating a powerful Sith Entity through which he gained visions of the future which he was able to use to persuade many on the Council that he was the Voice. Vowrawn is, understandably, initially suspicious until you save him from an assassin, and he eagerly commits to helping you. You encounter Draahg again in the Entity's chamber, now done up with fancy cybernetics. They don't help him as he actually dies when you defeat him this time. You free the Entity and arrange to meet Vowrawn in the Dark Council Chambers.
If you spared Overseer Tremel and Lord Rathari way back when, both of these characters appear to greet you in the antechamber alongside Vowrawn. You enter, interrupting Baras's latest attempts to finally proclaim himself as the true Voice. Your claim to be the Wrath is met with apathy by some, such as Darth Ravage, but others, such as Marr, are genuinely interested. A duel is arranged to show which one of you truly speaks for the Emperor. After numerous onslaughts, Baras's power fades and he is left helpless and forced to appeal for desperate help from his fellows. No-one rises to his defence. With one last claim of defiance, you stand with two options; kill him or spare him.
I must admit that as soon as I saw the light option to spare him I felt the impact of the climax was lessened somewhat. Yes, the canon option is to see him killed, and finally killing him is incredibly satisfying, but all the same; that you have a choice in the matter limits its impact in my eyes. If we had no choice but to slam our sabers into his gut then I would agree that the end is exactly as it deserves to be. The idea of the Emperor's Wrath even so much as contemplating leaving alive a false Emperor's Voice doesn't... it just doesn't sit right.
After Baras is dealt with, the Council addresses you appropriately, stating that you have free reign in the Empire. That is, as long as your aims and goals stay separate from those of the Council. As the thirteenth most powerful Sith in the galaxy, you leave the chamber in triumph.
The Sith Warrior story truly deserves its reputation as one of the two best stories in the game. Numerous cutscenes throughout are fantastic, the character and wit of the Warrior can be fantastic, and the notion of a guy you meet in the Prologue staying until he becomes the very end boss of chapter 3 is done fantastically well here. It allows you to pave a grudging respect or a building hatred for him across both first and second chapters and the third allows action on that.
The Warrior is truly a powerful individual, and the story demonstrates this to the extreme.