Head to the Marked location in the Catacombs. It helps to get there by using the Northern Underground entrance from the Windmill district and descending its shaft then climb into the Catacombs from the Quarry. Head to the North and into the water-filled room. Head to the Northern section of it using the wooden walkway. Dive off the end of it and head to the Northwest. It is only a short swim to climb the Bicorn.