The “Titan injection” is a fluid speculated to be synthesized from Titan spinal fluids, appeared at several plot points. When injected, it converts anyone (without the Power of the Titans) into a Pure Titan. The syringe was introduced in fragments of Eren’s memory, where Grisha injects him to pass him the power of the Attack Titan. Later, we see Rod Reiss attempt employ it to have Historia regain the Founding Titan from Eren. It was also used to save Armin who was mortally wounded, by turning him into a titan, then gaining the Colossus Titan. We also learnt much later that Marley uses it on convicted Eldians, turning them into Pure Titans to roam Paradis Island.