I'm literally quoting Dansuer at this point. Frostmorne "imprisons" your soul, even if you go off into the shadowlands, Frostmorne can draw any soul its killed from wherever it is in the shadowlands & bind it into a living corpse. But only because he had Arthas, he had a champion. If he deemed necessary, he could have taken control of Uther at any point. As that is how Death Knights are made. In fact if they wanted to blow our minds the twist could be that Arthas was actually protecting Uther from this fate by not turning him into a Death Knight.
Side note: We also see Ghosts of Uther in game at numerous points: They never explain this but I have to assume it's like how Druids can go to sleep & visit the emerald dream. When a soul in the shadowlands is dreaming, they're manifesting in the living world as a ghost. This includes when they don't know they're dreaming or dead. They'll never explain this properly but at least this theory makes sense.
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So I want to prepare everyone that Maiev got forgiven for killing a dozen night elves with a week in prison: So Sylvanas' punishment would equate to like 18 months in prison.