From Halopedia, the Halo wiki
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.
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.
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.
The Banished, a mercenary organization, had several legions who's Legionmasters report directly to a War Chief. They were composed of thousands of infantry, and were mostly disorganized autonomous packs free to plunder as they desired.
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