It would break an established trend of the things listed as trials actually being trials.
There is a definite precedent for them showing up again given that the Final Days would likely send the people of Thavnair into a panic and potentially provoke enough prayer to call a more authentic version of their primal into being...though we'd have to destroy them anyways because having gods bleeding the world of aether when it's already coming apart at the seams would not be favorable
I'm assuming it's probably underneath the former imperial palace as a nod to its location in FF II (The Emperor's castle transforms into Pandemonium in the endgame) and judging from the Tower of Zot there's an extremely extensive underground space beneath all of the towers.
I'm also feeling like the story is going to be less about Lahabrea still being alive in some capacity and more just cleaning up a mess he created eons ago that only ended up becoming a problem due to the second coming of the Final Days. It was his department that captured Twintania for research purposes, only for it escape and wreak havoc so perhaps there was a whole slew of captured terminus beasts and discarded creations that were too dangerous to be destroyed and were simply sealed away instead.
