The Imperial Legion was far more powerful and diverse in its capabilities before the current decline of the Empire of Tamriel, which began with the close of the Oblivion Crisis at the end of the Third Era.
At that time, the legendary and historic Mage's Guild was disbanded and its famed Arcane University in the Imperial City in Cyrodiil permanently closed. This is relevant to Imperial Legion history because until that time, the Mage's Guild met a longstanding agreement to the Septim throne by training Imperial Battlemages. These specially-trained soldiers were trained both as warriors and in magery, and uniformed exactly like other Legion soldiers except that instead of wearing helmets, they wore blue hoods.
These Battlemages were assigned to various Legion units, as well as to their very own Shadow Legion. Additionally, they held the exclusive honor of guarding the Arcane University itself.