The insinuation was that Thordan turned him into
aether which he then consumed to sustain his Primal 'Knights of the Round' form (ironically just like how Leviathan did the same thing to the Sahagin Elder whom Elidibus uplifted with the Echo) - Thordan outright states that, that souls are "just aether, regardless if they're mortal or Ascian" (paraphrased), suggesting Lahabrea was now dead and gone permenantly, burned up like so much fuel.
Elidibus's comments cemented that even further stating "I alone remain" ( of the Unsundered) in the stinger of ShB. So there is a lot of questions as to the new raid coming in EW showing Lahabrea's glyph... but... given what
Pandemonium was in past FF games (and it's etymology in mythology), it pretty much underlines that "death is manflu for Ascians". ;)