Ackerman

Elena Smith sacrificed herself to save the Survey Corps, and in doing so, awakened the power of the Titan that has laid dormant since after the King's Curse. More importantly, this Titan was erased from history and the memories of humanity. This Titan has a connection so strong to the Founding Titan that it may be the only key to stopping Ymir. But no one, not even any of the Nine Titans, not even the Founder itself, remembers who or what this Titan is. Not unless they come in physical contact with Elena, or her Titan itself.

After learning the truth of her Titan, Elena returns to her home, Liberio, in an attempt to save her Father from Marley by bringing him to Paradis with her. Marley finds out about her return, and capture her to prevent her from escaping. After enduring much manipulation and torture, Elena still refuses to help Marley in their military expeditions to take over the world, and most importantly, to take Paradis. So she uses her Titan's hardening ability to crystallize herself.

Eren Jaeger has awakened the Founding Titan and is protecting his home and seeking freedom by destroying the world. The rumbling has finally begun, and it has awakened Elena from her four years of slumber.

Elena must make contact with the Founder if she wants to stop the destruction. Deep down, however, she agrees with Eren's beliefs and ideology. She seeks the same freedom that he longs for.

But her Titan's connection to the Founder and to a certain bloodline prevent her from achieving that freedom.

[Action. Suspense. And most of all, romance. Levi x OC, Eren x Mikasa, among others.]

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This story takes place during Attack on Titan: The Final Season. More specifically, this story is based off of the manga. If you are not caught up to Ch. 135 of the manga, then I warn you: there are MAJOR spoilers up ahead.