Imperious Avatar Helmet is an epic-quality equipment item that is only available to Heroes and is sold by Requisitions-T2H. Once equipped, it unlocks the Avatar Bombardier head costume style.
Source
The Imperious Avatar Helmet is sold by Requisitions-T2H, a vendor found within the Hall of Heroes between the Meta and Magic Wings of the JLA Watchtower. The item costs $429 and 80x Mark of Conquest.
Vendor item
Style unlock
Upon equipping the Imperious Avatar Helmet, a character unlocks the Avatar Bombardier head costume style, which can be accessed at any time from the Style Window.