The Underground Chapel (地下礼拝堂 Chika raihaidō?) is located beneath a chapel in a village near the Reiss family estate, to the east of Orvud District. It is a sacred location and the walls are composed of a luminescent ore that is thought to have been created by some form of Titan power. It is here that the royal family's power is passed down from vessel to vessel as a ritual, for generations.
Above ground, it looks and functions like any other church. Below the altar, hidden by a rug, is a stairway that leads to a large underground cavern. The cavern was created by the First Reiss King, and contains hardened crystal walls that produce their own natural illumination.
The cavern below the chapel was created over a hundred years ago by the First King. Some time afterward, the chapel was built above it.
Throughout the last century, the Reiss family would gather inside the cavern in order to pass down the power of the Founding Titan every thirteen years. This was done through a succession ceremony where the current wielder would be chained upon an altar. Another family member would inject themselves with a serum and transform into a mindless Titan so they could devour their predecessor. Afterward, the successor of the Founding Titan would inherit the memories of the Fritz family and become bound by the First King's will.
On the day of the fall of Wall Maria, Grisha Jaeger confronts the Reiss family while they are praying in the Chapel. Grisha begs Frieda Reiss to use the Founding Titan to save the lives of the people of the Walls. However, she refuses and Grisha decides to steal the power, transforming into his Titan form. Frieda then transforms as well and battles Grisha, but she is not skilled enough to fully utilize her Titan's powers and is killed by Grisha and eaten. The rest of the Reiss family, with the exception of Rod Reiss, are slaughtered by Grisha as well, and the chapel is also destroyed by him. Rod Reiss escapes the massacre and covers up the truth by claiming that the chapel was destroyed by bandits in an attempt to avoid suspicion.
Shortly after the resolution of the Wall Rose invasion, Rod Reiss has Kenny Ackermann's Anti-Personnel Control Squad kidnap his illegitimate daughter, Historia Reiss, and Eren Jaeger. They are taken to the cavern beneath the chapel and Eren is chained in preparation for the inheritance of his Titan powers.
Hange Zoë learns about the chapel from a report commissioned by Erwin Smith. Doubting the veracity of the bandit cover story, Hange believes that Rod Reiss is hiding something there, and has Levi Ackermann and the remaining members of the Squad Levi ride to the chapel in search of the captives.
After having a vision of Frieda Reiss, Eren Jaeger wakes to discover himself gagged and restrained on a platform overlooking a large crystal cavern. He spots Historia approaching below him while wearing a pure white dress.
Rod Reiss and Historia place their hands on Eren's bare back, causing Eren to recall memories of previously being in the chapel. Rod attempts to coerce Historia into using a Titan injection and consuming Eren, regaining the Founding Titan for the Reiss family, who are the only ones who can fully use it.
After finding the passage to the cavern beneath the chapter, the Scout Regiment members ambush the Anti-Personnel Control Squad guarding the area. Despite the MPs' advantage in numbers, the Scout Regiment manage to drive back their opponents to their final defensive post.
Though initially receptive towards her father, Historia reconsiders upon remembering Ymir's words to her about living her own life. She breaks the injection vial before throwing her father to the floor. As she begins unlocking Eren's chains, Rod Reiss licks the remnants of the Titan serum and transforms into a massive 120-meter abnormal Titan that collapses the cave on itself.
Seeing the explosion from Rod's transformation, the Special Operations Squad moves further into the cave to find Eren and Historia. They help free Eren from his chains, but cannot locate a safe exit due to the falling debris. Finding a bottle from Rod's bag labeled "Armor," Eren decides to believe in himself and ingests the serum. After transforming into a Titan, he hardens his Titan body, leaving a stone shell and pillar-like structures that prevent the cave from collapsing further. Squad Levi is then able to secure an exit from the cave.
All remaining members of the Anti-Personnel Control Squad are killed in the collapse, save for Kenny Ackermann, who manages to survive, albeit fatally wounded.
After Historia is crowed queen of the new Royal Government, the shining stones are taken from the cavern beneath the chapel and distributed to the population to provide a limitless source of light. With this, manufacturing is able to proceed both day and night, allowing for jumps in productivity.