[MAJOR SPOILERS] On Souls and the Lifestream

[Tharne]

My guess is that the player character can't be counted like a normal individual, and thus our soul(s) are unique. I have the theory that it was our original Amaurotine self the one which summoned Hydaelyn (with a different method probably involving linking her to us, with no tempering like Zodiark did with the Ascians after being summoned) to counter Zodiark. We most probably abandoned the Convocation because we weren't okay with summoning Zodiark, and after the other 13 members kept sacrificing half the population to make him stronger, it makes sense that we tried to do something about it.

They didn't sacrifice half the population to make him stronger. The second sacrifice was to revive the Planet's life, yes the aether goes to Zodiark but it goes back to the land after that, he gained nothing in terms of power.
And might I also add that those sacrifices were volunteers, they didn't just kill people.