|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?, also translated as Titan power(s)) allows Subjects of Ymir to transform into Intelligent Titans known as the Nine Titans, regenerate, and access memories of previous Subjects of Ymir that held the power. If a Pure Titan gains the power of the Titans, that Titan will return to being a human.
In order to transform into one of the Nine Titans, a human with the power of the Titans 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.
Those with the power of the 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.
Those with the power of the Titans can regenerate their bodies, as long as they have the will to live. They are even able to grow back their entire head. Regeneration from severe injuries such as lost limbs can strain their powers and prevent them from transforming for a period of time; 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. The healing 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 inheritors play a significant role in passing these memories. Both the Reiss and Tybur families kept their Titans—the Founding Titan and the War Hammer Titan, respectively—within the family, and because of that, they were able to access memories going back by several generations, up to at least the Great Titan War. Other inheritors are shown to be able to see the memories of the previous inheritor at the very least. This process appears to fail when blood relations are interrupted or not present: Ymir and Armin Arlert did not receive any memories upon inheriting their Titans, in contrast to the Reiss family who seemingly gained their memories right away. Porco Galliard is unable to see the memories of his brother Marcel possibly due to Ymir being between them in the inheritance line, although Porco could see Ymir's memories.
Curse of Ymir
Each person who gains the power of the Titans is fated with the "Curse of Ymir" (ユミルの呪い Yumiru no Noroi?), which limits their lifespan to only 13 years after first acquiring it. During the last years of their life, their body will weaken until they perish. Both Eren Kruger and Uri Reiss were sick and appeared to have aged considerably during their final year.
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, as one of them had shown some disobedience to Zeke's orders.
The power of the Titans can be transferred to a new host, should the current host be devoured by a Titan. This allows an ordinary Titan to become a new human with the power of the Titans, and inherit one or more of the Nine Titans abilities as well as the memories of the previous inheritors. This has been the case with the Founding Titan for over a thousand years.
However, according to Rod Reiss, the power of the Titans resides within a human's spinal fluid, meaning that to gain the Titan's power, it is not necessary to eat the entire human, but rather a Titan can simply bite through the spine and ingest said fluid. Titan serums are, in fact, the spinal fluid of Titans, though study of the serums is difficult due to the serum's rapid rate of evaporation upon contact with air.
If an individual with the power of the Titans dies before it can be transferred, the power will be passed on to the first baby among the Subjects of Ymir who is born directly after the said inheritor's death, regardless of distance or relation to the previous inheritor.
- 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."