Titan injection

A Kyojin no kessei (巨人の血清) is an injection-based method of turning an ordinary human into a Titan. The authorities of Marley used the serum as a sentence against Eldians, particularly the Eldia Restorationists, to roam Paradis as mindless Titans for the rest of eternity. The Paradis Eldians of the Walls, having discovered the relationship between the serum with Titans and Titan Shifters, realized that if one of their own is lethally wounded, they can use it to transform them so that they may eat an enemy Titan Shifter and gain their power, and as a side effect, save their comrade's life.

Uses
While the exact chemical compositions of Titan serums are currently unknown, it is actually 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. Very shortly after this, the person injected generates a mindless Titan body. This new Titan will remain mindless until it has consumed a Titan Shifter, after which the person will be released from the Titan's nape with new Shifting powers. 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 was already a Titan Shifter, or it may have to do with the very nature of the armor serum.

History
The origin of Titan serums and their usage is unknown, but it is probable that this was the method of creating new Titan Shifters 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 his Titan armor abilities.

At some point during the ritual for Historia's transformation, Kenny Ackerman steals another serum from Rod Reiss. 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.

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 turn humans into Titan Shifters.

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 to Armin.