Made in Abyss: Dawn of the Deep Soul

On the fifth level, Riko, Reg, and Nanachi meet Prushka, the adopted daughter of Bondrewd, the man who previously experimented on Nanachi and Mitty. Although Bondrewd welcomes them as guests, he experiments on Reg, removing Reg's right arm. Prushka helps Riko, Reg, and Nanachi escape Bondrewd, but Bondrewd dismembers Prushka to make a "cartridge" out of her. (Cartridges allow Bondrewd to avoid the curse of the abyss.) After battling Bondrewd to retrieve Reg's arm, Riko discovers that one of the cartridges Bondrewd used in the battle is Prushka, who is technically still alive, but in a compressed, crystaline form. Further learning that crystalized humans are the basis for white whistles, Riko takes Prushka to be her white whistle. Riko then uses her new white whistle to launch the diving bell to descend to the sixth level with Reg and Nanachi, while Bondrewd watches them go.

Synopsis from Anime News Network