Aldrich Faithful is one of the new covenants introduced in Dark Souls III. Before you can join it, you have to locate the covenant item first. After defeating Pontiff Sulyvahn, head straight from the bonfire and towards the large area with all the dead giants. Past all of the dead giants, head up the stairs and into the sheltered room. Instead of continuing up the stairs where the Anor Londo archers are, search the left walls for an illusory wall. Hit the wall to reveal a ladder leading down.
Take the ladder down, and you’ll be in a watery area with two of the dangerous lightning dog enemies you first encountered on the bridge to the Boreal City. Facing the two dogs on either side, head to the far left corner of the room where you’ll find the covenant statue. Interact with the statue and choose to join the covenant, and you’ll be given the necessary covenant item.
Open your menu and equip the item, and you’ll officially be a part of the Aldrich Faithful covenant.
Be sure to check out our Dark Souls III wiki for more tips and tricks.