I would rule out the ascians simply because the emperor seems to despise them also I question elidubus' loyalty too since he seemed quite uncaring that the emperor would go after them.

That said the Sahagin priest beign effectively immortal makes me think that the echo is a possible blame for this event, I mean that scene of the leviathan story made me think that it was extremely important, it kinda foreshadowed what would happen to Lahabrea but the reanimation with the echo could've been important too