because of the impending Eighth Calamity and Zodiark's imminent rebirth, the Scions decide to take one last desperate gamble... by
bringing back Goddess!Minfilia from the First. However, in order to do that, someone is still needed on the First to finish the job of balancing out the Light and Darkness there, and we're 'volunteered' for the task, against our wishes and better judgement (explaining Derplander's depressed-vibes in the trailer). Hence why
we become the Warrior of Darkness, it's simply for pragmatic reasons. Because the best way we know to deal with any kind of aetheric abomination is a good weapon and a strong arm to wield it! Especially one enfused with the opposing element that it's aspected to!
Accomplishing that we return back to the Source.. and lo and behold.... the Eighth Calamity occurs, Zodiark is reborn... and it all goes to hell in a handbasket... but
does it? Oh what's this? Because the First wasn't able to be fully Rejoined (due to our meddling as a WoD), and with our new mastery over both the Light
and the Darkness, we become the
Warrior of the CRYSTAL, truly Hydaelyn's champion in every way. We smack Zodiark, Eldibius and all the other Ascians around and destroy them for good, the Eight Calamity ends and the Eighth,
Eternal Astral Era begins. Minfilia returns to the Planet as Hydaelyn's avatar.. and we all celebrate!