Because of the legion artifact questchain, you see him and Arthas inside the shards of the blade.
Arthas is beaten and you even see Ner'zhul leave think he even says he will be back.
And he might be a echo, but you can now also ask yourself if Uther was a echo in halls of reflection or do we now have two Uthers in the Shadowlands after the blade broke in WotlK?