The Sanctum is a location from The Legend of Zelda: Breath of the Wild. It is located inside the Hyrule Castle, in the Central Tower region. The Sanctum is the room where Link confronts Calamity Ganon. If he hasn't retaken all Divine Beasts, he will have to battle Windblight Ganon, Fireblight Ganon, Waterblight Ganon, and Thunderblight Ganon there.
During "The Champions' Ballad" DLC Side Quest "EX Royal Guard Rumors", an EX Treasure Chest can be found on the floor located above Sanctum which can be identified by a unique looking pool of Malice in the middle of the room. The chest contains the rare Royal Guard Cap which is part of the "Royal Guard" armor set. Link can climb the outside of the Sanctum to reach this floor which is considered part of the Sanctum though entering it will not trigger any boss battles.