Made in Abyss: The Golden City of the Scorching Sun

In a series of flashbacks, Wazukyan leads a group of pilgrims sailing to the island of the abyss before the age of the cave raiders. The pilgrims descend into the abyss with Irumyuui, a native girl. On the sixth level, they establish a village, but soon discover that the abyss is mutating and killing them. Irumyuui gives birth to a mutated baby every day, but each one dies quickly. Wazukyan discovers that the villagers can survive by eating Irumyuui's babies. One child does not die, but quickly escapes the village. In the current time, Riko, Reg, and Nanachi arrive on the sixth level and discover the village, including some surviving pilgrims and some cave raiders, all mutants. Outside the village, Reg meets Faputa, Irumyuui's escaped daughter, who was born with most of Irumyuui's memories. Reg blasts a hole in the wall so Faputa can enter the village. She then kills Irumyuui to release her from her pain and takes revenge upon most of the villagers.

Synopsis from Anime News Network