Mephisto is the end boss of Act Three, and can be found at the top left of the Durance of Hate Level 3. The fastest way to get there is to use the Durance of Hate Level 2 Waypoint and run clockwise, following the wall. For players with a solid understanding of maps, you will want to go Left of the Waypoint tile. Examples provided below of the direction you want to look for the exit tile.