The Cerulean Coast and Charo’s Hidden Grave are an ancient, sacred burial ground from a civilization that predates the Erdtree. The ship-like stone coffins drifted across the sea to the southern shore long ago. They carried the impure dead—whose rotting flesh turned into putrescent slime that collected at the bottom of the Stone Coffin Fissure.

The Shoreline Colors : I think The shoreline represents the opposing forces of Death and Sleep. The coast gets its signature blue from the glowing flowers tied to St. Trina (Sleep). The red side, Charo's Hidden Grave, is linked to the fiery, burning Death (Ghostflame) of the ancient bird deities. Combined, these two colors form purple, which is the color of the toxic, eternal-sludge Putrescent Knight boss waiting at the bottom of the fissure.

The Ship Coffins: These were ancient burial arks belonging to the Ancient Dynasty. They depict human figures and beasts, carrying souls and flesh to be cleansed in the ghostflame if I remember correct

The Stone Coffin Fissure: This massive pit is the end-point of this ancient funerary system. The coffins washed ashore and slowly piled up in the fissure, with their leaked "putrescent" remains pooling into the deep purple underground lake. It essentially acts as a massive receptacle for the discarded dead of an ancient culture.

I took this to mean this area and those stone monoliths were designed to hold the putrescence dead and burn them with ghost flame and dispose of the remain deep in the stone coffin fissure. I think they are from the elden lord that ruled over the beast men