Headcanon: Lahabrea isn't dead for good, just trapped in Nidhogg's eye. When we destroy the dragon once and for all and save Estinien, Lahabrea will escape and either A.) Return to wherever the main base for Ascians is in order to recover (possibly without our character knowing he has done so), or B.) Reveal that he is using Estinien as his new host body, meaning we have to save him...again.

Though this time we might not be able to do so without harming/killing him...though either way it will partly be because Estinien regained enough control for us to land the true killing blow.