Birthday Death Disease is a mysterious disease that occurs on the surface around the Abyss. A person falls horribly ill on their birthday, and by the end of the day they will have died, no matter what medical treatment they receive.
It is thought to be the cause of all the 2000 year old Praying Skeletons found around a burial tower in the 1st layer. Underneath the tower is a tomb with even more Praying Skeletons, dating back 4000 years; leading into a secondary tomb with skeletons up to 6000 years old.
Though the reason for these mass deaths is unclear, Birthday-Death Disease seems to be a likely candidate. The cause of the disease is unknown, but is thought to be linked to the Abyss itself. Those who don't leave the Abyss when it goes around can only pray to be spared, thus it is theorized to be the reason for the praying skeletons' positions.