Lahabrea truly
is dead. He isn't 'locked inside the Eye of Nidhogg', when King Thordan slashed him with Ascalon he was turned into aether which, being a Primal, Thordan
absorbed and consumed as energy to sustain himself. Thordan clearly states this in his big
'the reason you suck speech' to Lahabrea in that scene:

Originally Posted by
FFXIV Heavensward main scenario quest 'Heavensward'
King Thordan: "Your contempt for man has proven your undoing Ascian. As my first act as god-king, I do hereby sentence you to die."
Lahabrea: "What?!"
(King Thordan slashes Lahabrea with one stroke and destroys him)
Lahabrea: "Aaaaaaargggh!!!!"
(Lahabrea dissipates into his dark aether and coalesces around the Eye within the blade's hilt before disappearing, not the usual votex style warp out effect when Ascians teleport away, but instead he just... fades away)
King Thordan: "Gods feed upon aether. It matters not what form that aether takes. Ascian souls are no exception. With this power, I shall put an end to a thousand years of conflict."
King Thordan: "Be they sown by Ascian, dragon or Primal, wheresoever the seeds of chaos threaten to quicken, I shall excise them with my divine blade and bring order to the world."
Also considering this was pretty much the same fate that befell the Sahagin Elder way back in
Through the Maelstrom where he gained Ascian-like transcendence only to become a tasty aetheric snack for Leviathan and be absorbed as aether, one may think
Lahabrea's ultimate demise had been planned all along.
Either way,
like disco, it seems to me that both Gaius and Lahabrea are dead and they are
never coming back.