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|Karl Fritz used the power of the Founding Titan...to give birth to massive Titans. These were to act as both his shields and his spears.|
|— Willy Tybur recounting the past|
The Colossus Titans (超大型巨人 Chō ōgata Kyojin?) are Abnormal Titans similar in appearance to Colossus Titan, one of the Nine Titans. They possess the ability of crystallization and exist in the order of the millions within the three Walls.
In year 737, King Karl Fritz used the power of the Founding Titan to create and command tens of millions of these Titans to create the three Walls and protect the last Eldian territory on Paradis Island. The lack of sunlight inside the Walls has rendered them immobile ever since, although King Fritz threatened to release them and use them to flatten the Earth had Paradis Island come under attack.
These Titans are similar in appearance to the Colossus Titan in that both their surfaces are covered primarily in muscle tissue and lack skin (although the Colossus has exposed bone over the muscle tissue of its form). They have proportionally small eyes and lip-less mouths which expose their teeth. Their exact height is unknown, but seeing as they make up the Walls, it is implied that they are extremely tall.
Each of the Colossus Titans have the ability to harden and they created the Walls this way.
In the year 737, after King Karl Fritz abandoned the conflicts of Eldia causing the Great Titan War, he fled overseas with most of the Fritz royal family and remnants of the Eldian populace to Paradis Island, Eldia's last remaining undisputed territory. He used the power of the Founding Titan to create and order tens of millions of Colossus Titans form the three concentric Walls to protect the Eldians. The King sent a message to the people of Marley, demanding that the Eldians on Paradis Island must be left alone or else the Colossus Titans inside the Walls would "flatten the entire Earth."
During the battle between Eren and Annie Leonhart in the Stohess District, Annie attempts to escape by climbing the Wall but is stopped by Mikasa Ackerman. She damages part of the Wall, however, and a huge chunk is torn off, revealing the face of a mysterious Titan.
The missing chunk of the Wall is immediately noticed by Mikasa. The Titan within the Wall follows Mikasa's gaze, and Minister Nick calls for the immediate covering up of the Titan, as the sunlight eventually will enable its movement. After Nick ignores Hange Zoë's questions about it, he is nearly thrown off the Wall by her, but still refuses to reveal any details he might know because of the Church of the Walls' forbidden nature to disclose secrets.
In the Reiss Chapel, Rod Reiss explains how the First King used the power of the Founding Titan to form the three Walls which surround humanity's territory, though the Colossus Titans were not mentioned.
During the festival hosted by the Tybur family, Willy Tybur tells the world of the truth about the Walls, the Great Titan War, and King Karl Fritz. He reveals that due to the King's vow of pacifism preventing the Founding Titan from being used for war, the Colossus Titans in the Walls were never a true threat; however, due to coup d'état in recent years, the military of Paradis Island have overthrown the previous royal government and the King's ideology of peace. Now that the Founding Titan belongs to Eren Yeager, who is seemingly capable of using its powers despite not being royal and without interference from King Fritz's will, the world is truly in danger of the Colossus Titans for the first time. Eren himself admits this to be true.
In Rikao Yanagita's analysis of the Attack on Titan manga, an estimation of the amount of Wall Titans residing within the Walls is given. Using the dimensions given in Attack on Titan: Before the Fall, Rikao estimates that, if the Wall Titans are all approximately 45 meters in height with a proportional 13-meter shoulder width with all Titans lined up shoulder to shoulder, there are approximately 246,000 Colossus Titans within Wall Maria and a total of 585,000 Titans within all three Walls combined. This conflicts with information given much later in the manga in Chapter 86, in which King Fritz's ultimatum given to Marley states that he would unleash the "tens of millions of Titans that sleep inside the Walls" if war was declared against Eldia. It is unknown whether the King's claim was accurate to the Wall Titans' numbers or was mere hyperbole.