After defeating 
Rennala, Queen of the Full Moon , within the Raya Lucaria Grand Library in Elden Ring, Tarnished adventurers might notice a locked chest sitting near the Queen. Unlike other containers throughout the Lands Between, this chest is locked, preventing you from accessing its contents. There is no key nearby, as the solution does not lie within the world but in a particular NPC’s questline in Elden Ring.