Auxiliary and double-digit Legions
These form the bulk of the Imperial Legion's presence in the provinces. Legions above number ten rarely have any individual history or identity, and are created and abolished by the Emperor depending on the situation. The Legion presence in Morrowind is composed of several such legions, working closely together: the legion number of a soldier is simply administrative. Provincial legions mostly recruit from the locals, though their officers are trained in Colovia. The "true" legions consider these little better than mercenaries without proper training or discipline, which is why the player has to start at the bottom rank after coming over from Morrowind. City Guards are also counted among these by the true legions. Many secondary sites in Cyrodiil, mostly in the northeast, are occupied by auxiliaries.
The First Legion
Symbol: a stag's head (reference to their Falkreath origin and victory over the Reachmen)
Originated from the Falkreath army that Cuhlecain first brought together, and formed the core of the united Colovian forces during the Tiber Wars. After the war, the legion gets honorary first status and becomes home defense for Colovia. By far the largest legion, mostly occupied with periodic recruitment drives in Colovia. The First habitually sends out trained lower officers to other legions around Tamriel, explaining the prevalence of Colovian Imperials as Legion officers. The First legion has not seen true combat for a long time, and has grown soft as a result.
This is where the player starts out, as a soldier recruited by the First. Early quests are routine patrols, training, that kind of thing.
The (Seventh?) Legion
Symbol: the Snake
This legion traditionally defends the southern Niben and mouth of Topal Bay, and the highways through Blackwood, prison transports towards Argonia. They have a high opinion of themselves for defending the Niben, the most immediate invasion route towards the Imperial City. Based in a large fortress somewhere close to Bravil. The Seventh has seen a lot of action lately due to the acquisition of the Trans-Niben: Renjiira Krin insurgents, hostile Argonian tribes, rebellion in west-Leyawiin. As such, they are more battle-hardened than the other Cyrodiil legions, and have younger officers (and an unusually high chance for promotion). There are rumors that the top brass have gotten involved in slave trading and skooma transports across the Niben.
After training in the First, the player is sent over to the Seventh as a lower officer. Here, he distinguishes himself in combat/gets involved in a corruption investigation.
The Third Legion (The Ruby Legion)
Symbol: the Ruby of Emperors. Also, red coats.
The legion of honor, defending the Imperial City and the surrounding circle of defenses in the Heartland. The palace guard and personal guard of the Emperor are also part of these. The Third distinguished itself during the Moth Rebellion, and was offered the special honor of guarding the Imperial City. Mostly a ceremonial legion, and much smaller than the other ones. Their armor is more elaborate and expensive than that of counrty legions. Rather arrogant, but very well-trained. Ever since the Interregnum, the Emperor has grown distrustful of the Ruby Legion, preferring to rely on his Blades.
After the Seventh, the player is promoted to the Third as reward for valor in combat/to buy his silence after exposing corruption among the Seventh leadership. Quests here could involve high-level political things, or peacekeeping within the Imperial City.
Most other legions are permanently based in other corners of the Empire (the Empire being hundreds of years old, there's no reason why each and every legion has to be based in the old homeland). A couple of legions off on temporary assignments may have left a token force in their homebase, but these can be represented by auxiliary (vanilla) forces.
The headquarters of the Legion, based on one of the islands of the Imperial City. An enormous fortress comprising the entire island, includes the Imperial Prison, barracks, archives, map room, etc. All the top generals are here. Also the home base of the Empire's militaristic nobility: the knights of the Garland, the knights of the Imperial Dragon, etc. Defended mostly by the Ruby Legion.
This is the endgame for the player, where he becomes a commander/general/whatever, and is offered the chance to reform the Tenth Legion
The Shadow legion
Rather dramatic name for the Imperial Battlemages, a subdivision of the Legion. Only legionnaries and certified members of old Nibenese aristocracy have the right to train in the College of Battlemages. After the destruction of the original Battlespire, the College of Battlemages is based in a decommissioned Battlespire on Lake Rumare, close to Bastion Island.