Arguably the most famous and feared troops in the Persian army, the Immortals were the best troops the Persian army could field, and only ever took to the field when the King did.
The Immortals were so called because if their number ever fell below 10,000 through ill health or death they were instantly replaced, giving the impression that their numbers never diminished. The Immortals were the best the Persian army had to offer, and only ever accompanied the great king when he lead his army on campaign.
They wore the finest scale armor, made in the Skythian style, and were equipped with shields, spear and bow. They would also have carried an Akinakes sidearm. Their clothing would have been highly decorated and presumably colored with the most expensive dyes, such as yellow, blue, red and purple.
The Immortal commander carries a longer akinakes, in the Skythian style. His status is shown by his saffron-dyed, tiara-style soft cap. It's possible that the scales of his cuirass could have been plated with gold.
The musician bears the typical trumpet of the period. He has no shield, so probably would have retired from the front line when the unit came into combat. He still carries a bowcase on his right hip for ranged engagements.
The standard bears a golden symbol of Ahura Mazda, the god of Persia. From the standard, streamers fabric-dyed with the highly prized royal purple are hung to show the king's favor.