Harbor Gate Key [Special]
~ "Unlocks the gate to the harbor area on Cassar Island"

Once this item is acquired, you can open the harbor gate in the south west corner of Cassar Island, which grants you access to the Raft.

How to obtain:
Along the main quest line, after completing the Thief Citadel, Stella in Cassar will ask you to kill a monster that emerges from the well in town at night, between 00:00 and 00:31. Kill the boss (Sniper Imp) in the well, and then talk to Stella. She will give you the Harbor Gate Key and the "Well Boss Killer" trophy will be unlocked.