As Attack on Titan revealed the form and power of Eren's aquatic Titan, readers witnessed a horrifying truth.
Towards the conclusion of the Attack on Titan saga, Eren successfully convinces Ymir to transfer her Founding Titan power to himself at the Coordinate. The consequence is a global cataclysm. Wielding the power of the Founding Titan, Eren initiates the Rumbling, awakening an army of colossal Titans within the walls, initiating a continent-wide march of devastation to put an end to the world.
However, Eren's Founding Titan form deviates significantly from other Titan forms in the narrative. Final Titan Eren stands out due to his extraordinary appearance and colossal size.

The Founding Titan is one of the Nine Titans, being the first Titan ever created. Typically, only members of the Fritz or Reiss royal bloodline can inherit its power. By emitting a scream, the Founding Titan can generate and control other Titans.
After acquiring the power of the Founding Titan, Eren employs its capabilities to awaken the dormant giants within the three walls and commands them to march towards Marley, eradicating any life forms in their path.
Eren's Founding Titan differs significantly from Ymir's Founding Titan. While Ymir's titan form resembles most other titans in the story, Eren's titan is notably larger and more skeletal.
Final Titan Eren's legs and pelvis are underdeveloped compared to the rest of his body, yet he still towers over Titans within the walls.
While colossal Titans stand at 60 meters tall, only visible over the walls, Final Titan Eren's form emerges prominently like an imposing, menacing presence threatening everything.

Final Titan Eren boasts an incredibly broad chest cavity, extending several meters. It's so large that wall Titans can walk within it as they march across continents. The upper body of Eren is suspended by tendons connected to the spine, resembling a puppet controlled by strings.
Before transforming into Final Titan Eren, Eren's head was severed by Gabi. Although the neck was reattached through Ymir's power, it remains elongated, adding to its increasingly distinctive and peculiar appearance.

According to recent CGI images released, Final Titan Eren's form is nearly as large as the Shiganshina district where he grew up. The chest cavity of Eren's Founding Titan spans almost the entire width of the district, while its spine is long enough to stretch from one end of the district to the other.
Shiganshina lies at the southern end inside the Maria Wall - the largest and outermost wall among the three. Unfortunately, Shiganshina has been reduced to rubble by the Rumbling.