This achievement requires you to find the Heirs of the Sun covenant. It is found by praying at the Altar of Sunlight, which is located in Harvest Valley just before the entrance to Earthen Peak. It is in a cave to the left at the top of the stairs leading to the building/cave with the pots of poison and a few Undead Steelworkers. Before entering the building/cave, look for a path to the left, running above the poison gas below.
While at the Altar, you should also learn the "Praise the Sun" gesture there, to contribute to the Gesture Maestro achievement.