Reiss Chapel

East of Orvud District stands a small church, outwardly ordinary in look and function, where the Reiss family comes to pray. A rug beneath the altar hides the way down: a stairway descending into an enormous underground cavern carved out by the First King more than a century earlier. Walls of hardened crystal enclose the space and give off their own glow, lighting the cavern naturally.

For a hundred years the Reiss family gathers in the cavern every thirteen years to hand the Founding Titan forward. The ceremony chains the sitting holder to an altar while another family member injects a serum, becomes a mindless Titan, and devours the predecessor, gaining the memories of the Fritz line and the binding will of the First King along with the power. Uri Reiss receives the Titan from his father in that ritual and later passes it to his niece Frieda in the same place.

Shortly after Wall Maria is breached, the whole family assembles at the chapel. Grisha Yeager makes his way below and begs Frieda to turn the Founding Titan toward saving the people of the Walls; refused, he transforms, kills and eats her, slaughters every Reiss present except Rod, and destroys the chapel itself. Rod escapes and blames bandits to smother suspicion. Years later he has Kenny Ackerman's Anti-Personnel Control Squad abduct Eren Yeager and his own illegitimate daughter Historia, chaining Eren above the cavern floor for a new succession; touching his bare back floods Eren with memories of the place. A report commissioned by Erwin puts Hange onto the bandit story's holes, and Levi's squad rides for the chapel, ambushing and driving back the Control Squad guards despite their numbers. Below, Rod presses Historia to inject herself and devour Eren to reclaim the Founding Titan for their blood. Remembering Ymir urging her to live for herself, she smashes the vial and starts freeing Eren instead. Rod licks the spilled serum from the floor and becomes a 120 meter abnormal Titan whose emergence caves in the cavern. Trapped under falling rock, Eren swallows a serum labeled Armor taken from Rod's bag, transforms, and hardens his Titan into a shell and pillars that hold the ruin open long enough for Squad Levi to escape. Every remaining member of the Control Squad dies in the collapse except Kenny Ackerman, who crawls free fatally injured.

Once Historia is crowned queen, the luminous stones are harvested from the cavern and distributed among the population as a limitless light source, letting manufacturing run day and night and driving productivity sharply upward. Far away in Marley stands an altar much like the one under the chapel, where members of the Warrior Unit receive the power of the Titans through a rite of succession mirroring the Reiss family's own.