Titan Injection

A Titan injection transforms one of the Subjects of Ymir into a Titan; every other living thing is immune to it. Marleyan authorities regard the serum as Titan spinal fluid, though its full formula stays a mystery, and Yelena's later admission that Paradis lacks the materials and expertise to replicate it suggests the fluid is processed and mixed rather than raw. Exposure to open air destroys the serum within moments, so every dose lives inside a sealed vial.

The serum's history is bound up with power itself. The Reiss family almost certainly used injections to pass the Founding Titan between monarchs ever since Karl Fritz raised the Walls, the practice that carried the power from Uri Reiss to Frieda Reiss in 842. The Tybur family presumably kept the War Hammer Titan in the bloodline the same way, and Marley's Warrior Unit inherits its Titans through an equivalent succession ritual.

Delivery is usually a shot to the inner forearm or the nape, though swallowing the serum also works. Transformation follows almost instantly, producing a mindless Pure Titan that regains humanity only by devouring someone who holds the power of the Titans. Method matters: after Rod Reiss merely licks spilled serum and becomes a grotesque 120-meter monstrosity, Kenny Ackerman guesses that botching the procedure carries consequences. The resulting Titan's size can even be dialed in, as Sergeant Major Gross demonstrates by turning a captive into a Titan of just a few meters.

Vials come in varieties. Rod carries a standard serum alongside one labeled "strongest," which he claims yields a Titan built for battle, and an "armor" serum; when Eren Yeager bites into the armor vial, his own Titan gains hardening. The legible katakana on the three known labels read "Strongest Titan," "Armor Braun," and "Normal Titan." The serum's users split along a moral divide. Marley's Public Security Authorities weaponize it as a life sentence, dumping condemned Eldians on Paradis Island as eternal mindless Titans, the fate inflicted on Ymir and her fellow cultists decades ahead of Wall Maria's fall. The Survey Corps instead turns it into a last-resort medicine, transforming a dying comrade so they can consume a shifter and return human.

When Zeke Yeager betrays the Eldian Restorationists in 832, the captured rebels are shipped to Paradis and injected, but the Marleyan officer known as the Owl reveals himself as Eren Kruger and spares Grisha Yeager, later injecting Grisha so he can devour Kruger and claim the Attack Titan. On the night Shiganshina falls, Grisha slaughters the Reiss family in their chapel, then injects his own son; the syringe's ornate design later tips Eren off that his father stole it from the Reiss stockpile.

Five years on, Rod Reiss prepares a dose meant to make his daughter Historia into the Titan that would eat Eren and reclaim the Founder. Historia smashes the syringe instead, Rod licks the residue and becomes a deformed colossus, and Eren's desperate bite of the armor vial saves everyone from the collapsing cavern. Kenny Ackerman pockets a vial from Rod's bag during the chaos and, dying, presses it into Levi's hands.

That single vial becomes the series' cruelest dilemma at Shiganshina, where Erwin Smith and Armin Arlert both lie at death's door. Levi chooses Armin, whose Pure Titan devours Bertolt Hoover and inherits the Colossus Titan. Years later the Anti-Marleyan Volunteers hand the Garrison a stolen stock of serum, earning Dot Pixis's thanks and, given doubts about their leader, their own arrest.