The Shadow Legion is a division of the Imperial Legion consisting of the Imperial Battlemages. It serves in the Imperial Guard, but had close ties to the Mages Guild before the Guild's dissolution. While the Guild had the authority to send battlemages onto the field, often to deal with violent rogue mages, the battlemages ultimately obeyed the orders of their superiors in the Legion.
The original Imperial Battlemages were an elite unit separate from the Legions, founded during the early days of the Alessian Empire when the distinction between mage and warrior was more pronounced. In time, the idea of a "battlemage" became less novel, and Imperial Battlemages began to serve alongside the regular Legions as spellcasters. Regulus Tharn was paramount in reviving the tradition of the Battlemage during the Reman Dynasty. During this time, the Imperial Battlemages were further exalted with the creation of the "Imperial Battlemage" title, a prestigious position traditionally granted to a unit's leader. Not to be confused with members of the Shadow Legion, these Imperial Battlemages served on the Elder Council and advised the Emperor. Several historic figures have held the title, including Abnur Tharn, Zurin Arctus, and Jagar Tharn.
The Shadow Legion were once trained in the Battlespire, a battlemage academy located in a thin slipstream between Oblivion and Mundus, which was used as a testing facility. Apprentices were sent here to prove themselves. During the Imperial Simulacrum, the Battlespire was invaded by the forces of Mehrunes Dagon, who killed nearly all battlemages stationed there. The Daedra blocked access to the Battlespire through the use of a Daedric Sigil, which prevented the Shadow Legion from entering and reclaiming the academy. Two battlemage apprentices fought their way into Oblivion and followed the retreating army, eventually banishing Dagon to the Void and causing the destruction of the Battlespire.
Despite the destruction of their training facility, the Imperial Battlemages continued to exist decades later. They had a presence in the Imperial City, where they were assigned to guard the Arcane University. There they patrolled the university grounds and occupied an Imperial watchtower. In 3E 433, Watch Captain Hieronymus Lex briefly commandeered the Battlemages, along with many other members of the Imperial Guard, to search the Imperial City Waterfront for the Gray Fox, a notorious thief. In reality, the Gray Fox's appearance was a lie circulated by the Thieves Guild, who used the opportunity to pull off several major heists around the unguarded city; the absence of the Imperial Battlemages allowed for the theft of Hrormir's Icestaff from the Arch-Mage's Tower, which was later returned by the Thieves Guild. The theft enraged the Mages Guild, who demanded that Lex call off his search and return the battlemages to the university grounds. That same year, the Imperial Battlemages helped the Mages Guild to catch a rogue mage along the Gold Road, and helped fight against the Order of the Black Worm.
After the destruction of the Battlespire and the dissolution of the Mages Guild, it is not known where the Legion was based during the Fourth Era.