This covenant can be discovered in the Undead Settlement.
From the Undead Settlement bonfire, work your way through the building and out into the open where you'll see several enemies around a burning tree. On the opposite side of this tree is a building with a closed door- enter this building and you can fall through a hole in the floor to get into an otherwise locked room below. The Warrior of Sunlight covenant item is on a table in this room.
Pick up this item to discover the covenant and the achievement will unlock.