The relic can be found at the start of Chapter 5. When you first enter the fortress through a window by climbing around the outside, you will have to go outside again to proceed. When you break through the second window, follow the passageway to the right, as if you were doing the chapter as normal. You will see a doorway to the left. Outside, there will be a pirate standing directly ahead of you, and beyond him, the remains of a structure that looks like a room. To your left will be a wall of the fortress, with pirates on top of it, and a stairs leading up to the top. Kill all of the pirates and go up the stairs (you will have to do this anyway to do this chapter). At the top of the stairs, go left, towards the remains of the “room.” Jump onto the roof of the “room” and in one of the corners, you will find the Strange Relic.