Burn the Vhenadahl is a quest in The Darkspawn Chronicles DLC for Dragon Age: Origins. It occurs during the main quest Raze the Alienage. The Archdemon has issued a command to burn the Alienage to the ground. By using either Fire Bombs or the Hurlock Emissary's Fireball or Flame Blast spell, set the oil barrels and the Vhenadahl tree on fire.
After entering the alienage, proceed across the bridge to the set of closed gates. Standing nearby will be a Hurlock Emissary. Free up a slot in your party by killing one of your thralls and then Enthrall the Emissary. Take control of it and have it cast Fireball or Flame Blast at the oil barrels to the left of the gates.
Once the gates are destroyed proceed into the Alienage. There are three more sets of oil barrels that you will need to destroy (all will have a questmarker hovering over them): one is just inside the gates on the right side at the bottom of the steps leading to a building; one is in a rubbish pile southwest of the vhenedahl tree near a sloping roof; the last one is in a rubbish pile southwest of the vhenedahl against a building. You can burn these barrels in any order. Once the final barrel is destroyed, the archdemon will command you to burn the Vhenadahl tree. Aim a fire Bomb or cast a fireball at the base of the tree to set it on fire.
Along with several armed elves and Elders, Zevran will show up during the fight.