: The Ten Thousand
The Immortals were a corp of elite Parthian troops that served as the staple shock troops in any Parthian assault. They were made of ten thousand of the best Parthians, hence the nickname The Ten Thousand was derived, and no matter how many lived or died, this number was unchanging. It was this that brought the name Immortals, as no matter how many died, there always seemed to be a constant number of them, and also due to the formations they used which made it seem as if a fallen soldier instantly rose from the dead to fight again.
The golden age of Parthia was around 5th Century B.C., and the golden age of the Immortals was respectively at the same age. They were used to great effect in the peloponnesian wars, even against the fearsome Spartans. In fact, the Immortals were not unlike the Spartan hopliota. As Parthia expanded into an empire, it began to draft from its newly conquered lands, and its army ranks swelled. The Immortals were the most elite of Parthian soldiers, but no matter how good one of their draftees got, they could never swell the ranks of this elite corp. The Immortals were led by Hydarnes II, son of Hydarnes I, who hand picked the men himself; but these men were always from one of the original Parthian tribes. In this way, they are similar to the Spartan hopliota who were only from the original Spartan tribes.
The Immortals were very difficult to kill, although they did not wear anywhere near the amount of armour the traditional hopliota wore. The only armour they wore was a very light scalemail coat and padded leggings. All Immortals wore the tiara, a simple soft felt cap that was designed for distinction, and a finely embroidered, long sleeved tunic and usually wore light shoes designed to keep the feet cool while in the desert. Apart from this, nothing else was worn. The typical layout of the Immortals was a light wicker shield, a long spear, a small dagger hung from the belt, and a very heavy yew bow with a small quiver of cane arrows to unload on the enemy before melee began.
What the Immortals lacked in armour they more than made up for with dexterity and skill. Perhaps their greatest triumph was at the battle of Thermopylae in 480 B.C. Herodotus describes this battle as an absolute slaughter. According to Herodotus, a large Greek army had blocked off a passage and repelled the Parthian army as they tried to break through. Seeing that another assault would only end in disaster yet again, the Parthian commander called for the Immortals and sent them quickly around the rear of the Greek army under the cover of night. A heavier regiment would have been both too noisy and too slow to achieve this, but the Immortals did so without waking a soul. At day break, the Immortals charged from behind, and fighting a force that was, according to Herodotus, more than three times their number, managed to rout the Greeks and clear a path for the main Parthian army.
The Immortals lasted into the 3rd Century A.D.*, but after this they became relatively obsolete. The Parthians no longer warred with the Roman Empire or the Greeks, but the Byzantine Empire, and their Byzantine Infantry was far superior to the Immortals. Thus they were replaced by heavier troops, and the Immortals were disbanded into the regular Parthian army.
* There is no direct evidence referring to the disbanding of the Immortals, however after 3rd Century A.D., there are few references to them, and there are none what so ever during the Byzantine Persian Wars.
Note: Sources on the Immortals are relatively obscure and exact details on formations are virtually impossible to obtain.