This is an ancient Snake Talisman. You received it from one of the powerful Snakes in a tale you like to call:
It all started when Operative Grillo was called by Kalinda on Mercy Island about some Snake problems occurring there. Grillo sent you to speak with Mongoose, who told you about a new breed of Snake he had run into which was vastly more powerful than anything he had encountered before. You checked out the Snake pit he mentioned and found a nest of powerful Snakes who spoke of the return of Stheno and rise of the S'lisur. Operative Grillo mentioned how Arachnos came to the Rogue Isles and first encountered the Snakes, driving them deep underground.
Operative Grillo wanted you to know more about the history of the Snakes, and so sent you to speak with Regent Korol. Korol told you a story about how the Snakes ruled over the Rogue Isles and how early settlers paid homage to them. The Snakes unlocked the secrets of the earth and the world trembled at the thought of Stheno.
Operative Grillo spoke with the Arachnos Mystics and they mentioned how the Circle of Thorns may be keeping information on the Snakes as well. You invaded a Circle temple in the Nerva Archipelago and found the Lore of the S'silur, revealing how in ancient times a pregnant woman drank from the Well of the Furies and was transformed by it. She became known as the Gorgon Stheno, sister of Medusa. Stheno fled across the ocean in search of a place where she could give birth to her progeny and came to the islands where the Rogue Islands now exist. The birth of her progeny exhausted her, and so she retreated underground to rest and recover for centuries. Operative Grillo told you this information would be useful in times to come.