Now you are getting somewhere (Tbh, he is my favorite villain).
~Edit~
It make sense...
If Jagar Tharn comes back as a Dragon Priest, or Lich again.. His name might be Miraak (He was allied to the Emperor and a guide, right?).. Also the DLC's name is Dragonborn so it has to be surrounded on the character some-how... And Jagar Tharn might be it.. He wanting to kill us cause we are the Dragonborn (A possible Emperor in his eyes)..
Herma-Mora's Realm could be used to bring him back OR could be used to figure out where a long lost artifact is that we will need to defeat him.. OR could be used for us to learn about his past.. You know?
All of this can be connected... And we can use "Divarth Fyr" of the Telv Tower to get to HM's realm..
To this day we do not know exactly
where Tharn imprisoned Uriel Septim.
Only that it was "a dimension of his choosing".
Maybe he had a pact with Hermaeus Mora?
I always thought Uriels dreams about the Oblivion Crisis were the aftermath of his experiences during the imprisonment. Maybe he read the wrong books?

Of course that's just speculation, but quite interesting.