Titan injection

A  is an injection-based method of turning an ordinary human into a Titan, usually so that they may eat a Titan Shifter and gain their power.

Uses
The exact chemical compositions of Titan serums are currently unknown, but it is said to be primarily made up of human spinal fluid. However, 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.

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. The design of the syringe used for the injection was identical to that of the syringes used by the Reiss family, so it is likely that Grisha stole this serum from the Chapel.

Five years later, Rod Reiss prepares to use another serum for his daughter Historia. 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 labelled "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 is all that is necessary to turn humans into Titan Shifters.