This is a broken seashell taken from the costume of the hero known as the Sea Witch. You took it as a souvenir of a tale you like to call:
It all started when Lorenz Ansaldo asked you to solve a problem for the Family. A hero known as the Sea Witch was attacking various Family-owned cargo ships, the latest being the Carpe Diem. They've been having trouble tracking her down, so they decided to hire a freelance villain to help them. The only clue the Family had was that she had worked with a group known as the Legacy Chain in the past. Invading a Legacy Chain base, you interrogated their leader, Arcanist Stigand. He revealed the Sea Witch had a relationship with a Scrapyarder leader known as Iron Billy.
Rumor had it that Iron Billy was hanging out somewhere in The Crush on Sharkhead Isle. Interrogating a bunch of Scrapyarders yielded you Iron Billy's hideout. Fighting through mobs of his Scrapyarder allies, you managed to kidnap Iron Billy.
The Sea Witch agreed to a final confrontation on the Carpe Diem, the ship she earlier put out of commission months back. Fighting through her Legacy Chain and Scrapyarder allies, you finally took out this enemy of the Family.