Once you have acquired the Secrets of Farovia skill, you will eventually encounter the Ancient Mariner when you are travelling between Blue Crescent and any other island. When you find him you receive this message:
The Ancient Mariner gave you a gift: The Sans Culpra Scroll!Edit
You met the Ancient Mariner on your voyage and he believes you are now ready to explore destinations beyond Blue Crescent. He gave you the Sans Culpra Scroll as a gift for your dedication and excellence
You will now be able to travel to Sans Culpra.