'A Burden', Asgeir's personal journal entry, describes his encounter and "deal" with Jormag. The area is called Asgeir's Legacy, speculated to either the site where he A) originally battled Jormag, or B ) perished / vanished after returning a second time. Asgeir describes Jormag's voice as 'home', which may allude to further fascination with the elder dragon.
I suspect he may not be dead, and may actually be a future dragon champion boss. We've had this theme before, with Saul D'Alessio among others - legends thought perished.