The ancient lakes (Patreon)

A ritual going on about as long as the tribe has existed

Everyone in the village cannot die by old age or natural means but in exchange once a month 3 girls would be selected at random to be put into a cave and sealed where their legs will fuse together and arms fused back into their body with their face removed as well with a gag sealed inside the skin of their mouth. After unsealing the cave after a week, they will then be thrown into a lake where they will join hundreds and generations of their ancestors as immortal to suffer for eternity where their pain will feed the old gods.
The village people doesn't know why they must do this but only know the whole village will suffer a worst fate if they dont do this.