So the story starts off with the central premise that the main character and the Horadrim are basically enemies of the state. This is coming off the heels of what happened with Inarius (a character who feels like he was written because the writer really liked Hazbin Hotel and its themes, but that's a side tangent) and his death. Basically, Prava and the Kyoshad nation believes that the Horadrim stole a soul vessel and unleashed hell on Sanctuary. This leads to Kyoshad taking more extreme measures in how they deal with heretics or people who may have close connections to the Horadrim... and none of this is done by Prava by the way, this is done by some goofy Judge Holden type character who seems to only exist to prop up Prava as a character and make us like her for being the grounded lot... even though she literally screamed at us for being heretics back in Hell during vanilla which Donan (who is another character I have grievances with) stupidly allowed to live.
So we get a lead... from Prava

to go to some east asian (or maybe it's South American?) country to find this lead and he tells us that he's not seen Neyrelle in a while (we'll get to her, fuck). We do a bunch of boring, superflous combat encounters tied to Blizzard's trademarked quest design. We encounter Not Judge Holden who managed to capture our lead who was helping us kill a bunch of corrupt demons during those superflous combat encounters. He cut off his arm and he tells us that Neyrelle will face justice for her crimes and that we should just surrender peacefully. Our character refuses and she has a vision of Neyrelle chanting a hilariously stupid asian derivative speech... because I guess we as the player somehow care for this character. I've never had a game look me in the eyes and tell me "yes you care about this character!" before. I've never seen it done in any other forms of media. I mean, fuck, I don't remember Goodbye Volcano High doing this and that game was Sweet Baby Inc slop. But I guess I, as the player, care somehow... even though she's an idiot who ran off with a shard that could fucking wipe out life in a blink of an eye. And is ACTIVELY wiping out life in a blink of an eye right now with the corruption.
Judge Holden stabs us and we're thrown into a river, bleeding. We were stabbed in the stomach. Our character is thrust into deep water with crimson covering the screen. Our eyes are even open for dramatic effect. But we managed to get beached onto a shoreline of a river and some guy heals us through some east asian magic type shit. This is Eru. A character who seems to trapse between being a know at all and being a spineless cuck depending on whether the writer wants to throw some melodrama into the story for whatever reason. I blanked out these parts of the story because 1. it's boring and 2. it's mostly just kill shit and move on. Riveting quest design. His characterization seems to flip and flop depending on whatever the writer thinks is cool or whether the writer, who is probably a woman by the way, wants to wax siloliquies about a bunch of not asian philosophy that has helped her through her trauma. In fact, this whole story feels like the writer just wanted to explore aspects of herself and her culture that she comes from.
More boring shit later and we arrive to find Neyrelle at the Durance of Hate... somehow. I don't know if Mephisto lead her to this place or if she was manipulated into finding this place. I also don't really know what significance this place has in forwarding Mephisto's plans. Oh I know why! It's a reference to Diablo 2! But she's stuck in some weird trance like state and nobody can reach her. So we need another fucking macguffin to basically kick her out of this state. Not before Mephisto tells us how cool and evil he is. Once again, the writer telling me what I think about this character which is new. I don't remember any other media doing this.
Again, more fucking boring shit of jacked on tasks to find this place to find this macguffin or this person who can help us. Once again, I blanked this entire section because it's boring and forgettable and a slog to sit through. But we enter the "spirit realm" and it's falling to Mephisto's corruption... I think. It really makes me wonder why demons even need to invade Sanctuary at all if they can just build an even stronger army in places only special humans can access. Well it turns out the person we're supposed to meet got corrupted by Mephisto and he attacks us leading to his inevitable death. Also Eru and this guy, forget his name, were supposed to be friends or something. We steal his heart and we take it to Neyrelle to wake her out of her stupor. And this is the very moment the story goes from dull and forgettable to annoying and frustrating. This very moment is when the story shits the bed so hard, it breaks the floor beneath.
Neyrelle is probably my pick for the worst character ever written ever. Wuk Lamat is bad, but she's at least a cheery character who stays grounded and positive in the face of sheer danger. Wuk Lamat was just the result of some mad bastard wanting to write a dating simulator girlfriend. Neyrelle really feels to me in this expansion like an author insert and it's not hard to see why given that she looks almost like, coincidentally, one of the writers of Diablo 4:

Huh...
She's also the one who left after Vessel of Hatred's release. Very coincidental. But anyhow, as bad as it was to constantly "speak to Wuk Lamat" during MMO questlines in Dawntrail, just imagine you're trying to complete a dungeon with Wuk Lamat in the party and non-fucking-stop, she has an emotional breakdown about Zoraal Ja and his invasion of Tural. That's essentially Neyrelle's entire fucking mode of operation in the entirety of the expansion. Constantly stopping the fucking game and snapping the pacing in half just so she can throw a temper tantrum, have an emotional breakdown, angst out for no reason, and constantly ripping the controller out of the player character's hands just so they can give therapy speak to this totally not author insert.
I'm utterly suspicious Neyrelle was written solely as inspiration from this writer's "personal experiences" and "wanted to represent my culture through my own personal trauma and tribulations of mental illness".
So why does this make me so angry? Because it completely
STOPS THE FUCKING QUEST. You cannot progress any further until you stop and give Neyrelle a hug during a long cutscene or long conversation. What should take a minute of horse riding, a couple minutes of mowing down demons and skeletons, and maybe throw in a five minute long boss fight instead takes damn near twenty to thirty minutes. It pads out the length of these simple fucking quests and it basically derails the pacing.
Her characterization gets even worse as the possession starts to worsen. We go from having emotional breakdowns, to having literal fabric of reality breakdowns where her "trauma" and "mental illness" manifests into demons we have to kill. If you're playing this on a difficulty beyond whatever doesn't put you to sleep, these enemies are pretty hit spongy and they can take a while to kill. And she does this countless times where she'll scream and out comes demons that you have to fight. Sometimes special enemies which take forever to kill.
So during the quest of trying to find Akarat's tomb (this macguffin character who will somehow help Neyrelle and her attempt to contain Mephisto in the crystal who also ball washes her constantly with his dialogue), Eru gets kidnapped and we have to find him. A character I really could give less of a fuck because he either exposits information at me or throws a tantrum just like Neyrelle. Well he's been kidnapped by the Kyoshad knights that are under Judge Holden's guard. So we go and rescue him, not before Neyrelle keeps stopping the game several times to have a breakdown. We find Eru and, off screen, she kills off one of our literal who companions who I forgot the name of and has no personality besides just to step on Eru and be propped up to be more emotionally stable than Eru. Eru, losing another friend of his, but seems to forget we have a mission to stop this corruption, bitches out and tells us just to leave... which in turn causes Neyrelle to have another emotional disturbance that we have to correct. Oh and Judge Holden gets murdered by us too. Punctuated with the cringe worthy line of, I wish I was joking, "you were a tyrant and a horrible person, but you were at least honest about it". Our character literally says that out loud.
Now we have to go to Akarat's tomb with more boring quests and more breakdowns from Neyrelle. We find it, but we need Eru to come and open the way for us somehow. He shows up again, guess he got his wits about him, and opens the way and we go into Akarats tomb. More boring combat and boring fights later and we finally arrive at the resting place of Akarat who is somehow still preserved (like no signs of rot on his body at all). Why are we even there to begin with, because I guess Akarat is going to contain the crystal somehow. But then it turns out *gasp* Eru was actually working against us all along! He betrays us and steals Akarat's body with the crystal and now we're left to fight a manifestation of Mephisto... because apprently demons can just manifest out of the blue from these soul stones if they gain enough power. I thought the whole point of the soul stones was that they needed a host vessel to take possession of. We defeat him with Akarat's help, but this in turn kills Akarat. Or maybe it just depowers him or something. Akarat then, before passing on, has to say the most cringe worthy line of "I've died once already, but now I'm afraid!".
Then we go to confront Eru who refuses to tell us where the crystal or Akarat's body is because he's looking to "protect the spirit realm". Feels really out of character for a guy who just watched an entire realm get corrupted by the very Prime Evil he just out of the blue served to use this as a prime motivator to turn against us. Neyrelle out of anger knocks him out cold and then

wax siloliquies about how he's no different than us and we should just leave him to live with the consequences of his actions and find forgiveness. Because let's just forget Mephisto has a corpse (I guess demons now can just take dead bodies and possess them, this makes Diablo seem like an idiot for trying to possess Leoric's son in the first game), the main town we were answering to to find this tomb has no idea what's going on and could be surprise attacked at any moment, because let's forget killing him isn't even necessarily an option in the first place, why don't we just make him face justice and have him interrogated?
And that's the end of Vessel of Hatred. Just an agrivating and infuriating piece that was just the writer's exploration of her own personal self while we, the player, have to experience it. An ending that has to, of course, have Eru, the character people might debatebly like, become a monster in the end for no reason. Hey, you know what would be a better ending that's dark and fits the lore of Diablo? Akarat becomes weakened after fighting the Mephisto manifestation, Eru decides to take the soul stone and stab it in himself and demands Akarat seal him somewhere in the spirit realm, Akarat, with his remaining amount of power, does so and that basically forestalls the events until Lord of Hatred.