A Trapped Soul is encountered during the quest The Ascent of Arceuus. Players are requested by the Tasakaal to kill the Trapped Soul as compensation to help the citizens of Arceuus. Players can find the Trapped Soul by investigating the grave south-east of Mount Karuulm which will result in tracks. Investigating the plants and tree stump along the tracks will allow you to find the Trapped Soul. When found, you will receive a chat message: A trapped soul that was lurking in the plants jumps out and attacks you!
It can be fought in the Nightmare Zone after the quest is complete.