The Sylphid Path is Titan's Trials area D3. You can initially access Missions 48 (E4) and 49 (E5), both of which lead to the Titan's Throne. Once you have completed Missions 45–50, you can access Mission 51 from here (or any other "D" area). You'll find the marks at the far end of this looping path, which it itself crawling with various types of Goblins.