Shortly after we arrive in Oribos, before we're sent to Bastion to start our little mystery investigation through the Shadowlands, we get a cutscene that explains why the afterlife is broken, and all souls are currently going directly to the Maw.
But one explanation just raises other questions.
What we know is that when things are working correctly, the Arbiter sees every soul and understands it immediately, then sends it on its way to the proper afterlife. But in the midst of the stream of incoming souls, there was some red (in contrast to the tranquil bluish-white of the others) missile that struck her, knocking her out and causing the entire stream to go directly down (which, ok, bad design, guys.)
As I see it, there are two possibilities: the red projectile was either a soul or some kind of magical attack.
Now, the latter case leaves a ton of possibilities open - basically, anyone who can aim something into the Shadowlands could pull it off. This could still totally be the case, but I think it's more interesting if one individual's demise was what caused this.
But who?
Ok, let's talk timing.
Thanks to the tearjerkingly tragic Ardenweald Afterlives cinematic, we know that the system was still functioning when we did the Emerald Nightmare raid. Ursoc's demise there led to his arrival as a spirit in Ardenweald, ready to undergo the long hibernation that would bring his eventual rebirth - only for the anima drought to deny him the chance to be reborn or even live on in Ardenweald (oof, still hurts).
So we have a moment it could not have happened before (even accepting that time works differently in the Shadowlands, we're talking causality here, and if the story is to make sense, it has to at least respect that a little, unless we find out the Infinite Dragonflight is behind all of this (which... sign me up?)
Now, I believe that there's been mentions of the souls that burned in Teldrassil going to the Maw (which makes Elune's putting them to sleep before they could die feel like less of an effective act of mercy). That would suggest that this violation of the system happened before the War of Thorns.
So that means that something happened during Legion or in the between-expansion period after it. If my hypothesis that the red thing is a soul, whose might it be?
My initial guess would be Argus. Frankly, there are a lot of things we have yet to really understand about Argus. When we arrived on his planet, he was still just a World-Soul like Azeroth - which seems to be a kind of formless essence at the heart of the planet. The Legion had worked Argus over like crazy, as we saw that a giant chunk of the planet had been carved away, and it had been profoundly corrupted by fel magic.
But in the very last moments of the raid, Argus manifests as a true Titanic form within the Seat of the Pantheon, and we fight him while the uncorrupted (and in the case of Aggramar de-corrupted) Titans watch. It's his defeat that seems to empower the Pantheon to capture Sargeras and seal him on his throne (though again, we really don't know what exactly happened next - the one kinda-sorta mortal there hasn't been able to call home.)
But here's a question: do Titans go to the Shadowlands?
I believe there was either an answer from a Blizzard person in an interview or possibly a line in Shadowlands that suggests that the Titans are beings of Order, and that the material world is their realm (which is kind of interesting - if each of the 6 primordial forces has its own plane, with the Shadowlands being that of Death and the Twisting Nether being that of Disorder, is "Reality" the realm of Order?) Though Sargeras had slain Aggramar and the rest of the Pantheon way back in the distant past, they lived on, and while their essences were (with the exception of Eonar) recaptured by the Legion (as we saw in Antorus,) they were able to return to their Titanic forms by the Argus fight.
Man, it's a shame we didn't get more time to spend with them, because there were a lot of questions I'd have wanted to ask them. Also, I'd have really liked to be reminded what the difference between Norgannon and Golganeth was - after so long building up the ultimate divine pantheon of the Warcraft universe, we didn't really get any time to see them do anything individually.
Anyway, if Argus hasn't just been annihilated like Telogrus was (I maintain that that's the name of the World-Soul that Sargeras first destroyed when he found that the planet was corrupted with Old Gods, because it's heavily implied that the Void Elf starting zone is that world) would his soul go to the Shadowlands? Or would it persist, maybe in the Seat of the Pantheon?
Let's set him aside for a moment, though.
What about Kil'jaeden? Killy J even had a connection to the Shadowlands, as it was he who built the Lich King (out of, I believe we'll discover, pieces made by the Runecarver, plus one Orc soul). But that's also an interesting question: given that Kil'jaeden had spent 25,000 years as a demon (and a particularly powerful one at that, meaning he must have been saturated with fel energy - which is probably why he exploded and took out his whole flagship when he died) can he even go to the Shadowlands? My read of his death was that he faced true extinction, dying in the Twisting Nether as he did. It made his last words all the more tragic - that he had given up his humanity (er... Draenanity? No, hold on, Eredaranity?) thinking that Sargeras could not be opposed, and that it would be better for him and his people to be on the winning side, only to face oblivion.
Like Argus, I'm not sure that there is a soul of Kil'jaeden's to even go to the Shadowlands.
But here's, perhaps, an interesting option:
What about Gul'dan?
See, there's a problem with Gul'dan. His soul had already been sent to the Shadowlands.
Now, take this with a boulder of rock salt, because even in Warlords of Draenor - the literal time travel expansion - Blizzard was resolute in not actually making use of the time-travel elements in the story. As someone who has been fascinated by the Infinite Dragonflight since they were introduced in Burning Crusade, it felt criminal that they never actually made use of them within Warlords - the only tiny reference to them being Kairozdormu claiming he would "become infinite" right before Garrosh strangled him to death.
While I think there's a lot of merit to the theory that no one is supposed to go directly to the Maw - that everyone's supposed to get a chance in Revendreth - if there's any character in Warcraft who seems 100% suited for the Maw, it's Gul'dan (Cho'gall's another possibility).
(As a side note, in the Gul'dan Harbingers cinematic from Legion, I think most people would read, and perhaps the intended read is, that the gruff warrior guy who exiles Gul'dan from his home is just another toxic "warrior culture" orc who can't understand the value of people without physical strength, and that the shaman who shows him mercy is the wise one, even if it gets him killed. I kind of like the possibility that the warrior guy doesn't exile him for being weak, but because he understands the darkness within what was then a young man, and that the shaman is just being naive.)
So Gul'dan might have just gone to the Maw immediately. But what if, when we killed the second Gul'dan, it caused an error?
The inhabitants of Bastion have a vaguely mechanical vibe to them (same with the Brokers, who, by the way, I'm convinced are up to something profoundly shady, and also feel like they would get along well with the Ethereals) and so I could understand Gul'dan Mk. II showing up for judgment causing a major error.
Now, on the other hand, plenty of invading orcs from Alt-Draenor died before he did well before Legion. And I also wonder if the Shadowlands transcends timelines the way that the Legion does (which... I'm about 95% convinced was the Blizzard lore team just wanting to get Warlords over with and let Archimonde die for real-real).
So I'm not 100% convinced that it's him either.
Of all the things that happened in Legion, the one most closely associated with the force of Death was Helya and her Kvaldir. We now know that Helya is one of the many figures in league with the Jailer (likewise Mueh'zalla, who I think is the hardest dungeon boss in the expansion, at least on normal and when we're still learning the fights) and we were told during BFA (if you got a particular drop from an island expedition) that even in death, she's been more powerful than ever.
Naturally, I think we'll find out more over the course of this expansion. But I am happy to have some major lore to speculate about.