"Who, WHAT are you?" *main story spoilers*

[Alberel]

That idea had been discussed to death and it simply doesn't work. The Scions, who are dedicated to finding more about primals and where they pop up, would have noticed something was amiss the moment they saw you.

The primal theory doesn't work, sure, but we still don't know what an eikon truly is... That one would still be a possibility I think. The WoL's aether is noted as being unique by various characters, just not in the same way as a primal's.

what about the Warriors of Darkness then? Y'Shtola noticed something was off with that group.

Unukulhai's story suggests that the Warriors of Darkness may actually be the ones that destroyed his world. If true then they've been absorbing corrupted primal aether for a long time which would explain Y'Shtola's reaction.