In the Sanctum of Domination Cinematic, we see that the Jailer returns Sylvanas’ Soul at the end with a crystal resembling very much the same one he pulled and gave away to a smith when forging Kingsmourne?
Which Soul do you suppose that contains? Most obvious choice for me would be Arthas.
Arthas is the least obvious choice because his soul was good.
If he put Arthas soul in there Anduin would have been an even better person.
Arthas did nothing wrong until he picked up Frostmourne which was the moment he lost his soul. And even then, Arthas was still good enough to keep the scourge from annihilating Azeroth and cross the Jailers plan for him .
It’s definitely Arthas. We are dealing with a narrative team whose shocking twists are on the same level as ‘peek-a-boo’ to an infant, and at least the infant has the excuse of working on object permanence.