|What I saw...looked like the manifestation of humankind's anger.|
|— Mikasa on the Mysterious Titan|
The mysterious Titan (謎の巨人 Nazo no Kyojin?) is a Titan power that is currently inherited by Eren Jaeger. The Titan first appeared during the struggle for Trost and has become a symbol of humanity's newfound hope ever since.
Grisha travels to the Reiss family chapel in the night. Bursting in on the royal family, Grisha activates his Titan ability, and attacks the family, intending to steal Frieda Reiss's Titan power. He easily bests Frieda in combat and devours her, before going on to slaughter the entire Reiss family with the exception Rod Reiss, who escapes.
That night, after Wall Maria's breach, Grisha reunites with his son. In tears and on the verge of hysteria, Grisha gives Eren the basement key and forces him to take an injection that causes him to lose his memories. As he is injecting Eren, he tells Eren that he must get to their basement, and that Eren must learn to control his power in order to save Mikasa's, Armin's, and everyone else's lives. The injection causes Eren to transform into a Titan, and Grisha is devoured by his own son.
For five years, Eren Jaeger lived his life unaware of his possession of the power of the Titans until the year 850, during the struggle for Trost. Upon being caught and eaten by a Titan, Eren's injuries and burning desire to slaughter the Titans trigger his first transformation into the his Titan form. In a nearly mindless state, Eren wanders Trost as a Titan, slaughtering many Titans throughout the day all while avoiding bringing harm to the human cadets around him. His Titan eventually exhausts itself, and Eren emerges with healed wounds.
Completely clueless as to the nature of the Mysterious Titan's powers, the Trost Garrison takes Eren into their custody and questions him about his Titan nature. When it becomes clear that the Garrison soldiers present cannot be reasoned with, cannonfire is used against Eren, but he successfully accomplishes a partial transformation of his Titan and protects himself and his friends from the blast. Upon Commander Dot Pyxis' arrival in Trost, the military decides to make use of Eren's Titan to seal the breached gate of Trost.
Upon reaching the boulder needed to seal the breach in Wall Rose, Eren's next transformation goes awry when his Titan falls into a feral state and attacks Mikasa Ackermann before incapacitating itself. The Mysterious Titan lies dormant for a short time before his friend Armin Arlelt manages to communicate with Eren himself through the nape of his Titan, and Eren regains control. Carrying a massive boulder through the streets as members of the Garrison Regiment and Cadet Corps defend him, The Mysterious Titan seals the hole in the Wall, securing humanity's first victory against the Titans.
Some time afterward, Eren joins the Scout Regiment after his change of custody is made official in a military tribunal. During his first month in the Scouts, The Mysterious Titan is subjected to a variety of studies by Section Commander Hange Zoë. At this time, the Scout Regiment discovers the dependency of the Titan's power upon a fixed goal in mind before transformation in addition to the drawing of blood. After the month's passing, the Scout Regiment sets out on the 57th Exterior Scouting Mission. However, the scouting mission is sabotaged by the arrival of the Female Titan. Within a Forest of Giant Trees, The Mysterious Titan battles the Female Titan, but he is defeated. While Eren is later rescued from the Female Titan, the scouting mission is cut short and is considered a failure.
A few days later, the Scout Regiment corners Annie Leonhart, holder of the Female Titan, in Stohess District, and she transforms into the Female Titan once more as a last resort. After some hesitation, Eren battles her a second time with his Titan. When he is overpowered late in the battle, his Titan undergoes a sudden change and catches fire in a berserker-like fit of rage. The Mysterious Titan gains the upper hand over Annie, though the assault comes to a close when Annie uses her hardening ability to encase herself in a crystal-like substance to avoid capture.
The next day, Eren joins the Scout Regiment's efforts in dealing with the sudden Wall Rose invasion. At this time, Reiner Braun and Bertholdt Hoover reveal their respective identities as the Armored Titan and Colossal Titan. When the two transform into their Titan forms, The Mysterious Titan battles the Armored Titan. Despite initial difficulties, The Mysterious Titan gains the upper hand and nearly defeats the Armored Titan until Bertholdt's Colossal Titan cuts the battle short in a last-resort effort to capture Eren.
During the week after the invasion, the Scout Regiment goes into hiding as Military Police action against them dramatically increases. Hange Zoë runs experiments on Eren's Titan in the wilderness, where they discover that Eren is unable to harden his Titan body. Some time later, Eren is captured by the Rod Reiss, who desire to feed Eren to his daughter, Historia, in order to return the Founding Titan to the royal family. While captured, Eren gains memories of his father using his Titan to devour the Founding Titan, and slaughter the majority of the Reiss family. Eren also sees the moment where he ate his father to gain the Power of the Titans. During Eren's rescue, he ingests a bottle labeled "armor," and gains the ability to harden his Titan's skin in a manner similar to the Titans inside the Wall and Annie's Female Titan. Eren uses this new armor ability to protect squad Levi from the collapse of the Underground Chapel set in motion by Rod's transformation. The next morning, Eren uses his powers again to shove gunpowder filled barrels into the mouth of Rod's enormous pure Titan. The resulting explosion blows apart its body and allows squad Levi to finish the job.
The Mysterious Titan's hardening ability becomes an important part of Erwin Smith's plan to retake Wall Maria. Upon the Scouts arrival to Shiganshina District, Eren switches to Titan form and uses his ability to successfully plug the hole in the Wall, then reverts to human form in order to hide from the Warriors suspected to be preparing an ambush.