Titan injection

A Kyojin no kessei (巨人の血清) is an injection-based method of turning an ordinary human into a Titan.

Uses
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 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 Titan power, or it may have to do with the nature of the armor serum.

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

The Paradis Eldians have used it as a means to save the lethally wounded; using it to transform them so that they may eat an enemy with the power of the Titans and gain their power, and as a side effect, save the wounded's life.

History
The origin of Titan serums 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 the First King.

Story
On the night of the fall of Shiganshina, Grisha Yeager travelled 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 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 daughter Historia. Upon seeing the Titan serum 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 a serum 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 a serum 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 serum from Rod Reiss' bag. As he is dying, he is found by Captain Levi. Just before his death, he hands the serum over to Levi. It is later decided by the veterans of the military that Levi should be entrusted with the serum. This serum 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 serum, and Armin's mindless Titan consumes Bertolt Hoover, the Colossus Titan, and Armin inherits the power of the Titans.

Attack on Titan, The Movie: Part 2
The Titan serum exists in the same function as a means to turn humans into Titans. But unlike the original manga and anime series, the serum appears to be all that is necessary to grant humans the power of the Titans. Furthermore, in the movie, Eren's father appears to have been manufacturing Titan serums on his own, as opposed to the manga where it is implied that Grisha Yeager stole his Titan serum from the Reiss family.

Attack on Titan: End of the World novelization
The true nature of Titan serums are further explained in the novelization of the live-action films. Two hundred and nine years before the events of the main story, an unknown nation, the ancestors of the people who would raise the Walls, developed a serum-administered drug called "Jaeger" which granted humans the ability to turn into intelligent Titans. The drug was developed out of the need to work around international laws which prohibited the use of weapons with certain lethal potential, since biological organisms could not be considered weapons. With this, they were able to defeat an enemy nation in war.

However, this enemy nation reverse engineered the Jaeger drug and in turn created a new drug called "Wraith", which turned humans into mindless Titans. Rather than administering this drug via serums, it was instead used as chemical warfare on a larger scale, leading to the widespread creation of Titans across the world.

Trivia

 * While only the "Armor" vial read by Eren has been identified officially in the story, the labels of the three Titan serum vials that have been introduced each bear legible kanji:
 * "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.