Thordan has a number of seeming unresolved inconsistencies that could use some retro-polish, anyway.

Nabriales delivers a monologue about how he was tasked with finding a way to "merge man with the divine". He declares Ysayle the first successful test subject - that success blamed on the Echo.

Quote Originally Posted by Nabriales"
I suffered the overweening presence of Lahabrea that men might host the power of gods, only for you to undo my hard work.
Quote Originally Posted by Nabriales
「超える力」を持つ者への「神」の憑依。
Those with "the Echo" can host "gods".
Then on NicoNico Toukaigi, the FFXIV team hosted a trivia game which declared that only Thordan was a primal, and that he used this primal power to bestow upon the Heavens' Ward the blessing of the divine forms of the Knights of the Round. (Not to be confused with the blessing he holds over their heads to ensure obedience; his primal-specific word for tempering.)

This of course appears to be in direct conflict with Lore Book 1.

The ritual used to imbue the archbishop's mortal vessel with the divine essence of his ancestor followed the same principle as that employed by Ysayle and her summoning of Saint Shiva—a rite previously perfected with the Heavens' Ward and their primal incarnations of the legendary knights twelve.
These two seeming inconsistencies appear to back them into a corner. Either interpretation leaves wonkiness in its wake because Zephirin already has a primal form early in the story, and shapeshifts in and out of it.

Assuming Elidibus gave Thordan the Echo might clean up a lot of this - granting him the presumed ability to repeatedly summon his primal form without destroying his mortal frame or tempering himself - but there aren't any good reasons to believe that's true and the devs seem hesitant to confirm or deny. Still, it fits with the Heavens' Ward vanishing upon his death that he blessed them ... which implies this wasn't the first time he became King Thordan.

And Tsukuyomi would then be an interesting mix of Thordan and Susano solving one another's respective primal wokiness (while making a grand example by temporarily tempering and eventually killing the Echo-less Yotsuyu).

I don't even try to solve this one, anymore.