Brands of Imperium is a heavy armor set. When you take damage, you have a 10% chance to grant you and your allies within 8 meters a damage shield that absorbs 12040 damage for 6 seconds. This effect can occur every 15 seconds.
The Brands of Imperium set is part of the Imperial City DLC. It drops in White-Gold Tower in the Imperial City. Each set item is bound on pickup. You can acquire heavy armor, weapons and jewelry of this set in the Xivkyn motif style.
If you need more information about the Imperial City, make sure to check out the full Imperial City Guide