From Guild Wars 2 Wiki
Hint: Use the Gloominator to capture the darkness inside the cave under Cereboth Canyon in Kessex Hills.
— In-game description
- Drop down the waterfall with the vista north of Cereboth Waypoint — to the very bottom. Turn and face the waterfall. To the right, there is a small cave with River Drake in the entrance. Follow this cave inside up to the platform with the Veteran Cave Troll and use the Gloominator there.