BioThe 501st Legion "Vader's Fist", Imperial Army, the infantry of the Empire or the armed forces of the Galactic Empire, was a legion which was formed during The Fall of the Republic era, when Order 66; the Great Jedi Purge, occured before the Galactic Civil War.
The 501st Legion originally was known as the battalion known as the 501st Torrent Co. during the Clone Wars led by Jedi General Anakin Skywalker, and was formed when the entire Grand Army of the Republic was absorbed into what was left of the 501st Torrent Co.; hence becoming the mighty legion led by Sith Lord Darth Vader.The 501st consist of having a huge amount of Imperial Stormtroopers (Phase 3 Clone Troopers), small squads of Royal Guards for Darth Sidious ( "The Emperor"), an infantry of Shock Troopers (Sandtroopers) for the Imperial
Security, and an entire naval fleet of pilot squadrons.
'Darth Vader' (of course) was the leader of the army; while Darth Sidious was the Emperor of his very own Galactic Empire.
Originally, the Clones wore Phase 2 armor, and still had their original distinguished markings from when they were in their original units during the Clone Wars. They eventually began wearing Phase 3 armor, and no longer were they Clone Troopers of the Grand Army of the Republic, but Imperial Stormtroopers of the Galactic Empire.
The 501st Legion was defeated by the Rebel Alliance (or the Rebellion) which ended the Galactic Civil War and the era all together.
Fans often confuse the 501st Legion either being the entire Grand Army of the Republic, or the 501st Torrent Co. However, the entire Grand Army of the Republic was actually absorbed into the 501st Torrent Co. becoming the 501st Legion.
The Stormtroopers only had three main versions of Phase 3 armor as opposed to having a large variety of Phase 1 and 2 versions of armor.
Phase 3 Galactic Marine (Snowtrooper), Phase 3 ARF Scout Trooper, and Imperial Shock Trooper (Sandtrooper) versions of armor were the main and only modified versions of standard Phase 3 armor. (Aside from the pilots.)
The entire galaxy was actually tricked and taken over by Darth Sidious. Including Anakin Skywalker, who was seduced to the Dark Side.The 501st Legion was actually a battalion during the Clone Wars known as the 501st Torrent Co.
The Stormtroopers in the 501st Legion (Galactic Empire) are actually named after the Clone Troopers in the 501st Torrent Co. (the Empire's counter-part); to whom the word "torrent" means "fierce winds" which is what a storm has. Hence: Imperial Stormtroopers of the 501st Legion.
The 501st Legion Fan Organiztion's name derives from the US Army's 501st Bravo Co.; to whom several veteran soldiers joined the organization years ago, which is what inspired the trademark name "501st" for Star Wars, and the Empire to be known as the 501st Legion. (Aside from it's Clone Wars counter-part.)
Levi's 501 series jeans also inspired the name as well, yet the other units of Clone Troopers from the Clone Wars era were inspired by real US military units.
The legion itself was also originally based on the "Third Reich" or the Nazi Army.