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The serum is used to turn humans into Titans shortly after they are injected followed by a sudden flash of light.
Over sixty years ago, a cult was discovered by a force of authorities, and the man who gave "Ymir" her name claimed she had deceived him. Ymir chose to abide by her given name hoping to save the other members of the cult, but this proved fruitless and she was arrested with the other cultists. After being paraded around town and stoned by angry citizens, the cultists were taken to a remote borderline. Despite Ymir's pleas for mercy, she and the other cultists were transformed into Titans by the use of an injection.
When cornered by the Trost District Garrison concerning his newfound Titan power, Eren suddenly recalls the moment when his father met him in the forest after the fall of Shiganshina. Insisting that Eren must return to the basement of their home, Grisha gave Eren an injection which he said would damage his memory and grant him a new ability. Furthermore, his father claimed that "their memories" would help Eren in learning how to use his new power.