Death's Door

This can only be done after completing the game and gaining the ability to turn the world to night. After defeating the final boss, returning to arena will reveal a key left where he died - taking this key to the locked belltower in the Lost Cemetery will allow you to ring the bell and turn day to night. After doing so, head to The Stranded Sailor and head into the ship to find Jefferson in the fish bowl on your left. Speak to him and after he hops on your back this will unlock.