The following article contains mild spoilers from Chapter 2 of 9.2's main story campaign.
Soul Fragments in 9.1During A New Path, a chapter from 9.1's Chains of Domination storyline, we learned about souls that had been permanently damaged by Frostmourne — such as Uther Lightbringer's soul. When a Mournblade is wielded in anger, it has the power to shatter a soul into fragments. The Jailer held a shard of Uther's soul, which we helped to retrieve from Torghast. From Sylvanas and Vereesa Windrunner's story in Folks & Fairy Tales of Azeroth, we know that the Jailer has held this shard of Sylvanas's soul since her death — and that it may very well be why she went directly to the Maw — rather than Ardenweald — when she died during the short story Edge of Night. In the Sanctum of Domination raid finale, we witnessed Sylvanas finally defy The Jailer just before he left for Zereth Mortis. As a punishment, Zovaal returned a shard of Sylvanas's soul to her — the shard that was torn from her soul when she died. Read More in Wowhead's Cinematic Analysis of the Sanctum of Domination Raid Finale We learned a little more about fragmented souls in the aftermath of the raid. In a conversation with Uther at the end of Chains of Domination, he explained that his soul «was not divided into one part that was noble and another that was cruel» but rather that it was as if a portion of his soul «remained frozen in time» while the rest of his consciousness carried on.
<Uther reaches toward the wound beneath his chestplate.> When Arthas struck me down, Frostmourne shattered my soul. But it was not divided into one part that was noble and another that was cruel. Rather, it was as if a portion of my consciousness remained frozen in time while Read more on wowhead.com