Kind of a weird dude. Snappy dresser, but.
Dark Souls 3: Leonhard the Ringfinger and the Black Eye Orb
Leonhard is a mysterious NPC found leading against Lothric's throne in Firelink Shrine. He'll give you five Cracked Red Eye Orbs the first time you have a chat.
Obtain a Pale Tongue by invading and killing another player (or perhaps by picking one up near an optional boss in Undead Settlement) and return it to Leonhard to receive the Lift Chamber Key. Use this to access the Darkwraith under Tower on the Wall. Speak to Leonhard after killing the Darkwraith to receive the Applause emote and the Red Eye Orb; the latter allows for unlimited invasions.
Leonhard suggests you join Rosaria's Fingers. (Note: joining Rosaria's Fingers will prematurely end Sirris's questline.) If you accept the Red Eye Orb from him and join the covenant, you'll find Leonhard at the door to Rosaria's Bedchamber when you offer a Pale Tongue to Roasaria.
When you visit a third time, Rosaria will be dead and you can collect the Black Eye Orb from her body. Don't worry: you can still access covenant services at Rosaria's bed, and you can revive her later if you want.
You can use the Black Eye Orb in the highest room above the Aldrich boss arena, you can invade and battle Leonhard. You'll receive the Crescent Moon Sword and the Silver Mask, as well as Rosaria's Soul. Use this to revive her, or transpose into the Bountiful Sunlight miracle.