What is "zhayedan"

Zhayedan (Immortals) were warriors of an elite unit within the Sassanian army, numbering 10,000 men. They are possibly modeled on the former Immortals, who served the rulers of the Achaemenid Empire, and possibly wore the same clothing as their predecessors. These warriors bore some of the finest quality weaponry and armor of the entire Sassanian army. The Zhayedan were led by a commander bearing the title of "Varhranighan-khvadhay."