Thordan destroyed Lahabrea not by a massive amount of energy, but by using Ascalon to reduce Lahabrea to his aetherial form which he then simply
consumed him, turning him into a tasty aetheric snack. Thordan and the Heavensward had become the Primal Knights of the Round by that stage, and Primals require a source of aether to keep themselves anchored in the corporeal realm, consuming any aether they can find. Thordan himself even says this outright immediately after consuming Lahabrea.
This was hinted way back in 2.2's storyline where the same fate befell the Sahagin Elder whom Elidibus had supposedly granted 'mastery of the Echo' to - the Elder had become a being of pure aether, and Leviathan consumed him as aether accordingly, destroying him.
As Knights of the Round Thordan and his knights planned to consume the rest of the Ascians, the Dravanian Horde
and other Primals alike as 'God-King', except his plan however had one fatal weakness - he was up against that meddling adventurer who had made a career for themself taking down Primals like he had become (namely, the player
). Needless to say, he folded so easily.
As for how Varis and Garlemald will use their newfound synthetic-Echo power... it's hard to say. But one thing is clear is that it will not lead us into an alliance with them - Garlemald is
still our enemy, they are still allied with the Ascians, and in true FF style, it's likely once we get to Garlemald's capital itself, finally taking out the Empire's ruling regime will then reveal the real string pullers all along, the
real big bad, which is almost guaranteed to be the Ascians and their evil moon deity Zodiark.