You get the mark for using the fire archer to kill The Menacing Hydra in A Hydra's Den.
Note: The mark is granted to the whole formation, since Sept-Oct 2009.
For giving the time and energy necessary to defeat the scourge of the Den, the Powers have granted thee a Mark. You have been granted the Mark of Asceticism! You gain 734,400 experience points! You gain 39 practices!