(The "cultist" is actually an imposter named Ria. (A cultist is attempting to free the cult's prisoners. (Harbin, a guard from Sentinel's Watch was looking for missing villagers when he was attacked. It triggers by talking to an injured Harbin whenever he appears at the entrance to the Misty Valley from the Foggy Plains.
Side quests from the Adventure Seeker only becomes available after reaching level 60 and changing the difficulty to Hell I. If this side quest isn't available, you can pay the NPC gold to shuffle the zone and side quests or check again the next day until it appears. Waiting Doom can be unlocked by checking the available side quests from the Adventure Seeker. Waiting Doom Overview Basic Information Quest Type