So on the PTR as part of the Uther questline we end up sneaking into a part of Torghast where the Jailer keeps the soul fragments taken by Frostmourne. We’re there to retrieve Uther’a soul fragment and along the way we see many other important souls along with many forgotten victims.
This is a little confusing though because we know that when Uther was slain part of his soul got sucked into Frostmourne while the bulk traveled to Bastion after being judged by the Arbiter. We know it, and many of the other souls we see in Torghast were in Frostmourne during Wrath because we interacted with the shade of Uther and Arthas’s father on more than one occasion - so how did those souls get to the Maw and why does that matter?
The first part is simple - those souls are in the Maw because of us, or more specifically because of Torino Fordring. When we defeat Arthas, Frostmourne is shattered into shards by the Ashbringer, releasing an explosion of all the angry souls Arthas ever claimed with the blade that swarm their former master and leave him vulnerable to us to finish off. We thought that we were freeing those souls to move on to the afterlife, and in a way we were right, but we did not count on the Jailer or the existence of the Maw, and we had no way of knowing that freeing those souls from the blade would just funnel them into even more sinister clutches.
Why does this matter? Arthas, the Lich King, may have been evil, but those he slew and did not raise were allowed to pass on to the afterlife naturally, going to whatever award the Arbiter might assign them (though their brutalized souls would still have the wounds Arthas inflicted on them as seen by Uther.) This was because the fragments of souls Arthas had claimed were kept in Frostmourne - however, with the blade vanquished, this would no longer be the case.
Early on Blizzard established that there would not be multiple versions of an individual in the Shadowlands. The Gul’dan of WoD for example, would find his soul blended with the Gul’dan of the MU timeline as an example. This is important, this matters, because it explains why Sylvanas’s soul went straight to the Maw without being judged by the Arbiter. It may also explain how Zovaal made Sylvanas his agent in the mortal plain and why she would have no choice but to do his will.
Once the souls of Frostmourne were in the Maw, any other pieces of those souls left on the mortal plain would snap to those fragments on the Maw. When Sylvanas committed suicide in the Esge of Night, she unknowingly stamped a one way ticket to the worst hell in the Afterlife. (Which is kind of sad since if she had died at any point before Arthas was defeated her soul might have been judged appropriately by the Arbiter.) With the fragment of her soul in his possession, it’s uncertain if Sylvanas was threatened into compliance, or if instead the Jailer simply used that fragment of soul as a bargaining chip, but either way Sylvanas had no choice but to go along with whatever he wanted so long as her soul fragment was still in the Maw.
Arthas needed to be deee de aged, and to do that his greatest weapon had to be neutralized, but in doing so we handed Zovaal exactly what he needed to put his long standing plans into action and sunder the veil of the Shadowlands. We champions of Azeroth were complicit in everything that came after. By removing Arthas and inadvertently delivering Sylvanas to him, we set it all into motion.
The question now is, how complicit was Sylvanas? We know from the Uther quest line that those injured by Frostmourne have their memories and personalities twisted and darkened. We also know that those fragments of souls were how Arthas originally commanded those that he raised. Sylvanas is likely a puppet, but it doesn’t seem like she was fully dominated as Anduin is. Was she threatened into cooperating, or was she genuinely convinced? Is she being blackmailed with her soul fragment, or did Zovaal return it to her as a sign of good faith? Even if we destroy Sylvanas, as she is now, will she really be destroyed so long as that stolen fragment exists somewhere out there?
Regardless of the truth, we now have an important piece of the puzzle.