Titan injection

A Titan injection (巨人の注射) is a serum-based method of turning a Subject of Ymir into a Titan.

Uses
The injection only works on Subjects of Ymir and has no effects on other races. While the exact chemical compositions of Titan serums are currently unknown, it is considered by the Marleyans to be Titan spinal fluid. It has been shown that the serums quickly evaporate after coming into contact with air. For this reason, they are always kept in tightly-sealed vials. Since Titan corpses also quickly evaporate, it would make sense that their spinal fluid would do the same.

Titan serums are injected into the bloodstream, typically the inner forearm or the back of the neck, depending on the situation. Very shortly after this, the person injected generates a mindless Titan body. This new Titan will remain mindless until it has consumed an individual with the power of the Titans, after which the person will be released from the Titan's nape with the power of the Titans. An improper intake of the serum has the potential to yield disastrous consequences. Rod Reiss' oral ingestion of a Titan serum caused his mindless Titan form to be too massive for practical movement. However, Eren Yeager could safely orally ingest an armor serum without negative repercussions. This may be due to the fact that he already possessed a Titan power, or it may have to do with the nature of the armor serum.

The Public Security Authorities of Marley use the injection as a life sentence against Subjects of Ymir who break the law, including the Eldia Restorationists, to roam Paradis as mindless Titans for the rest of eternity.

The Survey Corps have used it as a means to save the lethally wounded by turning that person into a Titan, then having it devour another with the power of the Titans to restore human form.

History
The origin of Titan injections and their usage is unknown, but it is probable that this was the method of creating new Titan monarchs within the Reiss family since the time of Karl Fritz.

Story
On the night of the fall of Shiganshina, Grisha Yeager traveled to the Reiss Chapel to speak with Queen Frieda. His reasons were presumably unknown to the Reiss family. After battling with Frieda in her Titan form and slaughtering most of the Reiss family, he reunited with his son Eren Yeager and injected him with a serum, transforming him into a Titan.

Five years later, Rod Reiss prepares to use another serum for his illegitimate daughter Historia. Upon seeing the Titan injection syringe prepared by Rod Reiss for Historia, Eren notes the identical embellished design of the syringe to the one used on himself years ago, indicating that Grisha had stolen an injection from the Reiss family. After much discussion and contemplation, Historia ends up shattering the syringe and chooses to free Eren, who was the prepared sacrifice for Historia's mindless Titan. In desperation, Rod chooses to lick the exposed serum, becoming a monstrous mindless Titan. In order to save his comrades from the collapse of the Chapel's caverns, Eren bites into an injection vial labeled "Armor", granting him the ability to harden his Titan.

At some point during the ritual for Historia's transformation, Kenny Ackerman steals another injection from Rod Reiss' bag. As he is dying, he is found by Captain Levi. Just before his death, he hands the injection over to Levi. It is later decided by the veterans of the military that Levi should be entrusted with the injection. This injection later becomes a subject of conflict as both Armin Arlert and Erwin Smith are brought to Levi on the brink of death. Ultimately, Levi chooses to save Armin with the injection, and Armin's mindless Titan consumes Bertolt Hoover, the Colossus Titan, and Armin inherits the power of the Titans.

Trivia

 * While only the "Armor" vial read by Eren has been identified officially in the story, the labels of the three Titan injection vials that have been introduced each bear legible katakana:
 * "Strongest Titan" (サイキョウノキョジン) - The one Rod chose for Historia and which he ended up ingesting.
 * "Armor Braun" (ヨロイ ブラウン) - The one Eren ingested and which granted him hardening abilities.
 * "Normal Titan" (フツウノキョジン) - The one Kenny gave to Levi and which was injected into Armin.

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