Shadowlands Afterlives: Bastion - World of Warcraft
Uther the Lightbringer, Paladin of the Silver Hand, met an untimely demise … but his service is not done. When he passes into the afterlife, Devos – a Paragon of Bastion – takes Uther under her wing, only to learn that some mortal wounds run deeper than she could imagine.
Frostmourne was a mourneblade, having the unique ability to fracture souls when wielded with rage as Terenas tells us. As we see in the Afterlives short featuring Uther [see above], his soul is fractured by Arthas with Frostmourne. A part of his soul is drawn into the blade, another part entering the Veil and ultimately being carried by a Kyrian—possibly Devos—to Bastion and the Shadowlands. This happened during the Third War.
The remnant of countless souls that were contained in the blade when Frostmourne was shattered were lost to the Maw. Moments later, Uther descends with Devos to claim Arthas before he even has a chance to pass into the Veil or face judgement by the Arbiter, throwing him instead into the Maw directly to face his eternal torment.
According to King Terenas Menethil II’s spirit, there were thousands of souls contained within the blade. Countless souls were freed from the broken blade, but others were not so fortunate.[25] As a mourneblade, Frostmourne could shatter the souls of some of its victims into fragments when it was wielded with rage.[2] When this occurred, the blade absorbed only a portion of the soul to empower itself, while the remaining portion continued to the Shadowlands to be judged by the Arbiter as normal. When Frostmourne was destroyed, the soul fragments it contained went directly to the Maw to be hoarded by the Jailer[26] in Torghast’s Ossuary of the Banished.[2]
https://warcraft.wiki.gg/wiki/Frostmourne#List_of_kills
As for us being able to interact with Uther, I reckon would or will require some handwaving.
That said, it is directly referenced that Maxwell manages to reach Uther through the Light. And that might honestly be the key to the whole thing, considering we know that the Light is able to influence and enter the Shadowlands in various ways. Revendreth and the presence of the Light in the Ember Ward comes to mind.
This is nonetheless speculation, however.
Although his body was slain, it was not enough to destroy the great champion of Light that Uther was. People visiting his tomb in Western Plaguelands might be able to see the meaning of the words carved on the marker stone by his tomb, for his spirit does live on, appearing to adventurers, members of the Alliance and Horde alike. His ghost expresses wisdom; words of gratitude to those who have come to honor his memory, and words of forgiveness to those who would wish harm for him, even in death.
https://warcraft.wiki.gg/wiki/Uther_the_Lightbringer#Legacy
At the time of the third invasion of the Burning Legion, in the aftermath of Tirion Fordring’s disappearance during the Battle for Broken Shore, Lord Maxwell Tyrosus believed that contacting Uther’s spirit would allow them to discover the current whereabouts of the Ashbringer.[55] Thus, Maxwell and a Champion of the Light journeyed to Uther’s Tomb and entered after putting the nearby awakened ghosts to rest. Through the power of the Light, Maxwell was able to contact Uther, who revealed that they had to hurry if they wished to recover the Ashbringer and that Tirion still clung to life. Uther then departed, despite Maxwell trying to call him back.[56]
https://warcraft.wiki.gg/wiki/Uther_the_Lightbringer#Legion
The brief meeting with Uther during Legion makes it unclear if it was the fragment liberated from Frostmourne, the Kyrian portion of his soul being contacted, the Uther interacted with during The Burning Crusade, or even a previously unknown fragment.
https://warcraft.wiki.gg/wiki/Uther_the_Lightbringer#Soul_Fragments?