The tormented mask of Icthlarin is a headgear that can be purchased for 7,000 oddments on a rotational basis in the Marketplace.
It was originally a rare, limited-time Squeal of Fortune prize that required level 15 Defence to wear. Occasionally after killing a monster while wearing the mask, a lost soul would appear, which could be clicked on to gain Prayer experience equal to 48% of what can be gained from a small XP lamp. This could be done 20 times before the mask's charges were used up, and it would revert to the mask of Icthlarin, which has no function.