When you come to the guest room,get the second Letter from Volcano Manor.

Open the map.    Come to the red mark position,and select Yes.

After defeating the boss Rileigh the Idle, we got Crepus's Vial and Black-Key Bolt.

Rileigh the Idle - Elden Ring Raiders

Return and talk to Tanith, Volcano Manor Proprietress to get Serpentbone Blade.