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.

Legions

 * Dn'end Legion

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

Banished
The Banished, an organization of mercenary raiders, had several legions whose Legionmasters report directly to a War Chief. They were composed of around 9,000 to 20,000 infantry each, and were mostly large mercenary clans that implemented other smaller mercenary clans within them, occasionally mixing and sharing troops across legions. For example, smaller Jiralhanae clans such as the Exodus Guard and the Long Shields were known to have used forces from the Sangheili Let 'Volir's clan, and Let 'Volir's clan were known to have used forces from Jiralhanae clans.

Core Banished at the Ark

 * Unknown legion
 * Clan of the Long Shields
 * Let 'Volir's clan
 * Colony's swarm

Banished at Zeta Halo

 * Unknown legion
 * Lance of the Writhing Star
 * Exodus Guard
 * Bulwark of Bone
 * Warial's Wrath
 * Brother's of Undending Ire
 * Heart of Tala
 * Beholders of the One Vapor
 * Sclera of Chu'ot
 * Thieves of the Claw
 * Hordeworms of Svir
 * Warriors of Malaston
 * Valorguards of the Chosen
 * Bloodbrave Guardian of Suban
 * Kaepra's Grief
 * Legion of the Corpse-Moon
 * Clan of the Ravaged Tusks

Banished at Earth

 * Unknown legion
 * Human mercenary groups

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 Wars 2
 * Halo: Official Spartan Field Manual
 * Halo: Shadows of Reach
 * Halo: Divine Wind
 * Halo Infinite