The first Afterlives animated short focuses on Uther the Lightbringer in Bastion.
We see as Arthas kills Uther. Shortly after, Uther’s spirit is welcomed in Bastion as a Kyrian aspirant. His training reveals Uther’s soul was wounded by Arthas’ Frostmourne blade.
We also see how exactly one of the Paragons betrayed the Kyrian to form the Forsworn.
We also see the last seconds of Arthas in Icecrown, and what happened after his death.