From Guild Wars 2 Wiki
Radiating Brandstone are large chunks of Brandstone that appear prior to, and during the fight with the Branded djinn Vemyen. They need to be destroyed before they errupt.
Vemyen created the first one as a warning to his old friend Ziya the Radiant to stay away due to his corrupted nature.
- One appears during the first event, while three spawn once Vemyen reaches 25% health in the second event.