This ashen place is grave of Wyrm. Once told, it came to die. But what is death for that ancient being? More transformation methinks.
The Wyrms were ancient beings who pulled bugs into their thrall. They were larger than Bardoon, though they had fewer limbs. They also had the ability of foresight, meaning that they could predict the future to some extent. One of these Wyrms died in Kingdom's Edge, transforming into the Pale King. A Lore Tablet in the Fungal Wastes remarks on him having prescience as well.
There is a reference to another being called the Blackwyrm in White Defender's journal entry. This creature shares the word "wyrm" in its name; however, it is unclear whether it is related to the Wyrms or not.
At a certain point, all Wyrms died out or were wiped out from Hallownest.
The former body of the Pale King, the Cast-Off Shell similarly has a horned crown. The ash falling in Kingdom's Edge is moult that originates from this decaying corpse. At the very end of the Cast-Off Shell, the King's Brand can be acquired by the Knight, marking them as King.