Welcome to the Blades of the Darkmoon.
Dark Souls 3: Sirris of the Sunless Realms
Warning: in any build of the game prior to v1.03, touching Sirris's summon sign will cause a crash.
You first encounter Sirris when you return to Firelink Shrine after chatting with Anri of Astora and Horace the Hushed at the Halfway Fortress. She is not friendly - initially.
Once you have Sirris's respect, you can summon her for three boss fights: Abyss Watchers, Deacons of the Deep and Lothric Younger Prince. If you join Rosaria's Fingers at any point, Sirris will become hostile; she'll no longer be available for summoning, and her questline will end prematurely.
Win Sirris's respect by collecting the Dreamchaser’s Ashes near the Old Wolf of Farron bonfire and returning them to the Shrine Handmaid. On your next visit to Firelink Shrine, Sirris will appear and grant you the Darkmoon Loyalty emote, which you'll need for the Blades of the Darkmoon covenant.
Encounter Sirris again near the Irithyll of the Boreal Valley bonfire. Rest at the bonfire, then return to the bridge guarded by a wolf beast and look for her summon sign. Activate it to be transported to her world, and aid her in defeating invading dark spirit Creighton. Return to Firelink Shrine and Sirris will give you the Silvercat Ring and a Blessed Mail Breaker.
After defeating Aldrich, return to the Pit of Hollows bonfire and activate Sirris's summon sign to defeat Holy Knight Hodrick - the real one, not the phantoms you may have encountered earlier in the game. Sirris will pledge her loyalty to you after this. (This also unlocks the Mound-makers covenant for those who missed it earlier.) Return to this area after travelling away to collect Hodrick's set.
Speak with Sirris in Firelink Shrine and she will express her satisfaction and readiness for death. Sometime after this, search the ledge between Firelink Shrine and Iudex Gundyr where a grunt is staring out into the distance to collect the Sunset Shield. Later, Sirris will die, and the Sunless Realm set will be available at the Shrine handmaid. At this point, the Sunless Talisman will appear on the same ledge where you got the shield.