Altar of Sunlight

Location

Found in Lothric Castle, behind a locked door next to the lift to Dragonslayer Armour. The door can be opened by climbing a ladder on the side of the building and entering it from the roof. You can then open the door from the other side.

Usage

It is used to level-up in the Warriors of Sunlight covenant, by offering Sunlight Medals.

Interacting with it allows the player to learn the Praise the Sun gesture.

Information here is accurate for version 2.15.