Legion

A legion is a generic term for a type of military unit. The term was originally used to describe an infantry unit in Ancient Rome; the general size of these units fluctuated during the military history of the Roman Empire between approximately 4,500 and 5,200 soldiers, the equivalent of large regiments or brigades in the UNSC Army and Marine Corps.

Covenant
In the Covenant military, a legion was a large administrative grouping of warriors commanded by a high-ranking officer. Legions were expected to operate as a combined-arms force. While typically non-permanent formations, warhosts that proved their skill in battle were occasionally adopted by legions. Legions are commanded by Sangheili generals. The Dn'end Legion was a respected legion of the Covenant.

Warrior crèche
A warrior crèche is a type of legion supposedly staffed by young warriors, though they could still attain considerable distinction in combat. Warrior crèches are carried aboard Covenant fleets should any ground actions be needed.

Warrior crèches

 * N'Nono Warrior Crèche
 * E'Toro Warrior Crèche
 * R'Lan Warrior Crèche

Banished
"The second was a horde of mercenary raiders called simply the Banished."

The Banished, a mercenary organization, had several legions whose Legionmasters report directly to a War Chief. They were composed of thousands of infantry and were mostly disorganized packs with a fair ammount of autonomy, free to plunder as they desired.

Legions

 * Atriox's Banished at the Ark
 * Clan of the Long Shields
 * Legion of the Corpse-Moon
 * Clan of the Ravaged Tusks
 * Keepers of the One Freedom

Forerunner
In translated Forerunner records, the term legion is used in reference to a type of Warrior-Servant organizational unit.

List of appearances

 * Halo: Ghosts of Onyx
 * Halo: Cryptum
 * Halo: Silentium
 * Halo: Broken Circle
 * Halo: Ground Command
 * Halo: Official Spartan Field Manual
 * Halo: Shadows of Reach