The Sith Inquisitor is another class whose story has received criticism for being "boring". Going into this critique, this stance seems to me to be one which is more privy to points-of-view more so than any other, because whilst most stories can appeal based on mission objectives and character, this story features history and archaeology extremely heavily, more so than anything else.
The prologue starts with you arriving as a slave to the Sith Academy on Korriban. You and several others are pushed aside, both literally and metaphorically, by a Sith Pureblood named Ffon. Double "F"s are a thing in Sith names, it seems. Because of your slave status, you are constantly being berated by your Overseer, Harkun, and must prove yourself against overwhelming odds. He informs you and your fellow acolytes that a Sith Lord named Zash is seeking an Apprentice, and you are the 'lucky' ones who are chosen to be a part of her pick-a-mix, if you will. You're duly sent all over Korriban to recover fragments of artifacts, speak to a crazed Sith prophet, and interrogate prisoners. Each time you excel, but most of these are just "Fetch/Talk to" events. Nothing truly taxing. You actually do end up opening an "unopenable" Sith Holocron just by blasting lightning at it, something which nobody has ever apparently thought of. Whether this is just idiocy on the part of everyone else or just a hint at your untapped potential is not truly made clear, but it is interesting to note that you managed what no-one else did.
Your trials end when you're sent into the Tomb of Naga Sadow in order to awaken a ferocious Shadow Assassin ("Dashade") named Khem Val. You actually have the advantage over Ffon, as he was sent on ahead just before Zash arrives and explains that the artifacts you brought back actually showed how to properly access his chamber. Whilst you would hope to encounter a perplexed Sith at the Chamber's entrance, you sadly don't find him and progress through to free and pummel the Dashade into submission. He, in turn, helps you vanquish a Terentatek and collect a Star Map. Returning to the academy, you find Ffon and the Overseer moping over the failure to collect the Map; he promptly takes it from you at Harkun's urging, but is unable to convince Zash, who arrives to collect it. She promptly executes him for his failure and summons you to her office, proclaiming you as her new Apprentice.
You're then sent to Dromund Kaas, where Zash's rival, Darth Skotia, makes himself known to you, warning that you are stepping into uncharted territory. Zash, when you relay this to her, reveals that she has plans to do away with Skotia, but it would require great coordination. His Trandoshan bodyguards and cybernetics must be overcome through use of an ancient Idol and a Technology Spike, respectively; the Trandoshans are killed when they betray Skotia to support you, but the Spike makes him vulnerable and so he dies. Zash is elsewhere, making her own presence felt at a social gathering. This means that, when her superior Darth Thanaton contacts her to pinpoint Skotia's death on her, she is able to prove that she was not responsible and is elevated to Darth-hood.
You, meanwhile, are sent to clear a Force Spirit away from a chamber which holds a complete and powerful Artifact belonging to Tulak Hord, one of several pointed out by the Star Map. This Force Spirit, however, is more familiar than it would seem; it transpires that he is a Sith Lord named Kallig, a contemporary of Tulak Hord, and that he is your ancestor. You don't unfortunately have time to grasp the severity of this, as you have to return to Zash in good haste with the Artifact and pretend to have vanquished Kallig. She gushes over you and proceeds to send you to the standard chapter 1 planets to recover the other Artifacts, promising that they will make you more powerful than you can possibly imagine!
Much like the Bounty Hunter, even though you have exactly the same mission objectives for each planet (Locate the Artifact), each time it is different. Balmorra's is in a toxic-gas-filled pit and so you must concoct a chemical which grants you temporary immunity to such gasses, akin to the native Colicoids. Nar Shaddaa sees you constructing a Sith Cult in order to challenge another Cult, the leader of which possesses your prize. The notion of a Sith Cult is fascinating, and the idea of it being on Nar Shaddaa of all planets is even more so. Tatooine has you help a Pirate Captain, Andronikos Revel, get back at his former crewmates who left him to die having all gone rather peculiar due to proximity to the Artifact.
Alderaan, keeping in line with the Knight and Consular in terms of internal affairs, sees you locating a Jedi from house Organa who broke off a marriage arrangement with a noble from house Rist - literally the only time Rist is seen as something other than a house of assassins - in order to pursue the Jedi life. She summons him back at your behest on the pretence of resuming their love, and then you can either take her with you or just kill her. Either way, Organa-jedi will try to kill you, so there is no difference in outcome depending on alignment. You then travel to the Elysium and locate the artifact in a secret Republic cache, using his security codes.
In among all of this, Kallig tells you to be wary of Zash; he guides you to reclaiming his Mask ("Countenance"), which would protect you from Zash's Force Powers - although it doesn't have any "Equip" benefits to this effect - and eventually his own Lightsaber. Armed with your ancestor's tools, you're ready for whatever Zash has planned.
I mentioned in the Hunter review that BioWare really knows how to make you hate someone; Zash, for many, is one of those who goes against this. She is easily very likeable and you don't get any real sense of imminent betrayal - you can choose to distrust Kallig on basis of paranoia - until you meet up with her at the end of chapter 1. One of the more surprising betrayals in-game for quite a lot of people, but nothing compared to another, far less pleasant, surprise concerning her appearance.
It transpires that Zash has aged drastically due to her constant drawing on the Dark Side of the Force, to the extent that she looks like a walking shrivelled corpse. She had been using a good portion of her power to make herself appear to be younger, to the effect that she looks 30. The artifacts which you recovered were tools for her own usage; the ritual which she wants to perform would allow her essence to enter your body and assume control of it, with one teeny little side-effect. You would die. Not wanting this, you of course fight her, although she proceeds to begin the Ritual. Khem Val, however, charges and gets in her direct path. Zash now has control of your Dashade, although she is now bound to you and can't kill you as the binding of the Dashade prevents such events. Khem also has control; his personality and hers take sporadic turns in controlling his monstrous form.
Zash's disciples arrive on the scene and, since Khem takes control of his own body again, Zash's true fate is unknown to them; they proclaim that she is dead and that you take her stead. It just so happened that she made you a Lord before her untimely "demise", in order that she would live her "new life" as a Lord rather than a meagre Apprentice, so you are now in a strong position in the Sith Hierarchy.
Chapter 2 begins with a summons to Dromund Kaas by Zash's superior, Darth Thanaton, who is now your superior. He commends you on despatching Zash and offers to help you on your own path to power, beginning with a simple task of reclaiming texts from a tomb. However, the tomb is inhabited by the Sith Spirit whose body rots in the coffin. He, under the impression that you are his traitorous Apprentice, tries to kill you but is stopped by Kallig. After lecturing you on "Tomb Safety 101", he informs you that Thanaton has disposed others who he despised in the manner in which he nearly killed you. He leads you to the tomb of Lord Ergast, who discovered something known as "Force Walking"; binding ghosts to you and taking their power. If you bind a ghost willingly, you don't receive as much power as you would if you force it to be bound against its will. You bind Ergast's spirit and the spirit of the vengeful Sith you met earlier, recover the texts, and return to Thanaton. He, surprised to see you, simply knocks you out with another powerful ritual. The ghosts, however, keep you alive, and now you need to find more ghosts to make your power all the greater!
Taris sees you chasing down a Jedi Apprentice by the name of Ashara Zavros ("Zavros"? What's next, a "Zalek"?), whose Sith ancestor is plaguing the Republic, so she accompanies you on the promise that you will vanquish the ghost. However, she is shunned by the Jedi when they discover her in the company of a Sith, so now she must come with you as an Acolyte-of-sorts. You bind the ghost and move on. On Hoth you are allied with the truly delightful Talos Drellik, an eccentric Reclamation Service agent, and together you track down the tomb of the Sith Lord Horak-Mul. He's actually willing to be found, as he hates the frozen wastes of Hoth; in order to "meet" him, you must first destroy the remnants of his rivals, much to Talos's horror. After everything is done to his satisfaction, you find him in the Star of Coruscant amid a horde of Kaleesh, bind him, and move on.
Armed with these ghosts, you return to Dromund Kaas, using a key stolen by Zash's former Apprentices before their untimely demise on Quesh to access Thanaton's private chambers. He doesn't fight you himself; he sends his own Apprentice to fight you whilst he sits in a Force Barrier. When you come to confront him, the ghosts in your body rebel, and Ergast reveals that binding too many ghosts at once results in fatality. You are thusly forced to be at their mercy whilst unconscious. You now not only have to stop Thanaton, but you also have to save your own life.
Much like the Trooper's story, the ultimate goal - in this case, Thanaton - is set aside for others - in this case, healing yourself and controlling the ghosts. Whereas the Trooper's goal-shifting didn't really work properly, this time it does make sense, as you can't face Thanaton without a whole body or mind. Zash informs you that Thanaton is onto your ploy and has hidden books by Sith dealing with these phenomena in his own private Library. You learn from those that Belsavis holds some machinery which can remake the body and that Voss has powerful rituals to heal the mind.
Belsavis actually is very interesting for a change, but only at the very end. You learn that the Rakata essentially created a whole myriad of species which are abundant in the Star Wars universe, notably Twi'leks and Zabraks (although other Expanded Universe material actually suggests that both of these species existed before the Rakatan Infinite Empire), and that she can essentially re-make your body from scratch using genetic codes. This event is later referred to on Rishi as a "Rejuvenation".
With this done, you move onto Voss, but not before you are contacted by an Imperial Moff, Pyron. He has been itching to complete the Silencer superweapon project but Thanaton - who is now a Dark Council Member - had shut it down due to his steadfast belief that no technical marvel could match the Force. You have to find some codes which are in possession of another Cult on Nar Shaddaa; the cultists in question want to lead your cult in place of those elected by Zash, and they can either be granted this request or simply mind-tricked into joining it themselves. You see the weapon completed and destroy a Republic Fleet and, if you so choose, the Imperial ships with whom they are engaged.
Voss is where you literally do battle with the Ghosts in the spirit realm. You can even bind a Voss ghost whilst in the Shrine of Healing! Eventually, you learn that there is a Gormak who has perfected some sort of mental-healing ritual, and together you pacify the Ghosts and grant you pure control of your body back. In return, the Gormak is transported from Voss, which breaks all sorts of Voss-law, but you don't have much time to care for such things; it's nearly time to go take on Thanaton.
Before you do, however, you're summoned to Korriban, as since your Apprentices were killed on Quesh, Harkun has been scouring for a good replacement. He presents a Twi'lek who is promptly killed by a Kaleesh named Xalek (close enough). After either pacifying or killing the spluttering Overseer, you're off to Corellia; Thanaton has challenged you to a "Kaggath", an ancient Sith traditional duel of power-bases, and you have no choice but to accept. Your tasks on Corellia are to therefore create and strengthen your own power base while challenging and steadily dismantling Thanaton's own.
Thanaton attempts to shut down the Tram Lines, but you get them working again, so he goes to destroy a fuel depot which your fleet would use, and you barely manage to stop him. You then go to destroy a shield-generator for his base, using pheromones to attract escaped Predators from Axial Zoo as a cover. However, Thanaton has in turn attacked your allies and is mocking you for being unable to defend them - although he doesn't manage to kill Pyron! With both of you at an impasse, he proposes to meet for an honourable duel in the Government District. You and he have a banter, and he sends his cronies forward. After defeating them, you and he recite the Sith Code (completing it will impress the traditionalist Thanaton), and duel there and then. Before you can land the killing blow, Thanton literally takes off and flies out-of-reach. You're informed that he's likely running to the Dark Council for aid, so you're back off to Korriban.
Of all the chapter 3 conclusions, I would argue that the Sith Inquisitor gets the best in terms of pay-off. I've complained how other characters such as Jun Seros and Rogun the Butcher weren't given the end worthy of either of them, but with Darth Thanaton the opposite is true. He actually does give a very decent fight, and the cutscene where you showcase your power over the ghosts and overpower him are amazing. After he is soundly defeated and actually left crawling back to his fellow Councillors, his neck is actually snapped by Darth Mortis! You don't even get a choice to spare him; he's viewed as worthless and promptly killed. Additionally, because you defeated a Dark Council Member in fair combat, you are elevated to his position and granted the title of "Darth".
It doesn't end there, though; after meeting with your followers on Dromund Kaas, you convene again with the ghosts. If you bound them by blood-oath (the "willing" option), two of them take off whilst the others opt to stay with you, but of course if you bound them by force, (the "unwilling" option), you can absorb their spirits and powers once again. And so you prepare to take the responsibility as one of the twelve most powerful Sith in all the galaxy.
All in all, the Sith Inquisitor story is one where personal tastes form a lot of likes and dislikes. As mentioned earlier, there is a heavy dotage on history and archaeology, so if these aren't your natural spheres of interest I can see this not being the most exciting of stories for you. Personally, as someone who has an absolute passion for these sort of things, I absolutely loved the story and learning so much about Sith history and culture.
Darths Zash and Thanaton are among the best of the bosses you can find in one of these stories; Zash, because she's actually likeable and her true nature is so horrifyingly unanticipated, and Thanaton because there is literally no choice but to see him killed, and by his own fellow Councillor, no less. His end is probably the most satisfying in the entire game simply because it is un-marred by chance of his being spared. Based on these two alone I would definitely recommend the Inquisitor story as one to not be missed.