Achievement name: The Great Escape.
This is awarded for getting out of the Maxos Temple. After you have defeated Amdusias, leave his chamber and take your first left. This area is blocked by a yellow/green barrier until you kill Amdusias, but once it is open go down it and there will be a cut scene with Damien. Once this is over he will leave, but a couple of his goons will appear and fight you. Once they are dead you can enter the portal which will trigger the achievement.
Note; there is a teleporter shrine in the Maxos Temple, but using this to leave the Temple will not give the achievement.