|What are you? A man? Or a Titan?|
|— Kitz Weilman to Eren Yeager|
The power of the Titans (巨人の力 Kyojin no Chikara?) is the ability which allows Subjects of Ymir to transform into one of the Nine Titans. Unlike Pure Titans, these Titans are intelligent, controlled by the human inside.
In order to transform, the human must suffer an injury that draws blood (often self-harm) while having a solid goal or belief in mind at the moment of injury. Being injured by another person is also sufficient. Once the transformation is initiated, a burst of energy is released, followed by the generation of the Titan body and a release of hot steam. After transforming, the human body rests within the nape of the Titan's neck as the center of control. The human body is also merged with the Titan's flesh inside to varying extents, most frequently on the area of the cheeks below the eyes.
If a Titan form is pushed to its limit, the human will emerge from the nape of its neck, sometimes in a semi-conscious or entirely unconscious state. If a Titan form is not exhausted, then its human form can emerge from the Titan form in full consciousness. In some cases, a human is able to partially emerge from their Titan body without fully leaving the nape. After the power of the Titans is used, oftentimes the area around the emerging human's eyes is marked by dark lines, which resembles the Titan in their possession, for a short period of time.
The ability to transform into a Titan can be hindered by injuries inflicted on the human body. A human can transform into a Titan with injuries as severe as a missing limb, but only if a previous transformation has not occurred recently. If a human emerges from their Titan form and receives severe injury, this person cannot transform into a Titan again until said injuries have healed.
Inheritors of the Nine Titans appear to be resistant to the normal absorption that creates mindless Pure Titans. However, this does not mean that they are immune to it. In the case of Eren Yeager, it has been shown that repeated transformations in a short period of time lead to decreased control over the Titan form, as well as diminished Titan size and power. As such, it seems that there is a limitation on how long a human can remain in a transformed Titan state before risking absorption. The length of time spent in a controllable transformed Titan depends on how skilled the human is, although even the most experienced can become incredibly fatigued for as long as several days after repetitive use. The exception to this appears to be the Cart Titan, which is known for its great endurance. Pieck is able to stay in her Titan form for at least two months.
Inheritors also gain their Titan's ability to regenerate, as long as they have the will to live. They are even able to grow back their entire head; however, regeneration from severe injuries such as lost limbs can strain their powers and prevent them from transforming for a period of time, and healing takes much longer as a human than as a Titan. The time it takes to regenerate depends on the extent of the injury and how focused the individual is on healing that area. This healing ability is not absolute, however, and can be overwhelmed if the injury is severe enough. It is also possible to delay the healing, which Eren uses to infiltrate Marley as an injured soldier. The only surefire way to kill a human with the power of the Titans is through severing the brain's connection to the rest of the body, although it is possible to survive even this by transferring the mind to the rest of the nervous system.
Those who inherit the Nine Titans are also able to see the memories of previous inheritors to some extent. Blood relations between the inheritors of a Titan play a significant role in passing these memories. The inheritors of the Founding Titan from the Reiss family are able to access memories going back by dozens or perhaps hundreds of generations, while the current inheritor of the Founding Titan, Eren Yeager, is unable to see further back than his father's memories and on only one occasion saw back into the memories of Frieda Reiss who preceded Grisha. Other inheritors of the Nine Titans are shown to be able to see the memories of the previous Titan at the very least. However, this process appears to fail when blood relations are interrupted or not present, as Eren Yeager and Armin Arlert did not receive any memories immediately upon inheriting their Titan forms, and Porco Galliard was unable to see the memories of his brother Marcel, though he could see the memories of Ymir who succeeded Marcel and preceded Porco. Sometimes, the new inheritor will have dreams of a previous one, such as the case of Armin dreaming of Bertolt Hoover's final moments.
Royal blood allows the "full use of the power of the Titans." This is common knowledge in Marley. Only descendants of the royal family can fully utilize the Founding Titan's powers. Zeke Yeager, a royal from his mother's side, inherited the Beast Titan, but thanks to his royal blood, he can mimic some of the Founder's abilities, such as creating and controlling Pure Titans and even making them move at night using only moonlight; no other Beast Titan had these abilities. Unlike the Founding Titan, however, the control over Titans is not perfect: the Gluttonous Titan had shown some disobedience to Zeke's orders.
Curse of Ymir
Each person who inherits the Nine Titans is fated with the "Curse of Ymir," which limits their lifespan to only 13 years after first acquiring the power of the Titans. During the last years of their life, their body will weaken until they perish.
- In the August 2016 Issue of Bessatsu Shonen Magazine, Isayama was asked of what is it like to come out of a Titan's body. He stated, "Just think that it's like coming out from a sauna."