From Guild Wars 2 Wiki
Hint: Use the Gloominator to capture the darkness in one of the test subject chambers in Vexa's Lab in Fireheart Rise.
— In-game description
- Use your Gloominator inside the middle one of the three chambers that spawn enemies in the first room of the Vexa's Lab puzzle.
- This must be done in a fresh instance, as the plasma bars open up only briefly while the enemies are spawning. In case the event chain of the lab has already progressed, it needs to be brought to completion, and then wait for the chain to start again.
- However, if the player has a Raptor mount, it can be used, with good timing, to rush into a chamber and attempt to capture the gloom through this method.