Curate, connect, and discover
Leave my crown on the branch to the east, where the last ray of light shines at high noon; rest my sword behind the two rocks, where the moss grows green and soft. And I will always be there to guide you, among the souls of our forefathers, in this Sanctum of Kings, Temple of Death.
-> ATKB Masterpost
(Small spoilers ahead in case anyone is actually reading this AU and cares)
Crows burn the bodies of their dead to free the souls from attachments as far as possible and allow them to pass beyond the veil more easily.
Kings and Queens however are oath-bound to the land upon coronation and once they die, their crowns and blades are brought to a sacred cavern beneath the highest hill, in order to bind their souls there so that they may be called upon and provide wisdom and advise to their successors.
Nobody who is not a crowned royal is allowed to enter to sanctum and nobody who has entered is allowed to ever speak about it with anyone who hasn't.
In the night after a coronation the older/ruling kings/queens show their chosen successor to the sanctum and decide where they want them to hang their crown/place their blade once the day comes. In order to complete their anointment, a newly crowned ruler must submerge in the waters—this will bind their soul to the land for eternity. This is why becoming a king/queen of Karasuno is binding for life and can not be reversed.
The cavern also has a much older purpose: It serves as temple and resting place to the Angel of Death, whenever they aren't inhabiting a vessel (which they current are -hint hint-). The Angel, who embodies the concept of death, is worshipped by many although barely anyone knows what the being really is and that is is far less omnipotent than commonly believed.