A Legion is a division of Elite warriors. They are most likely led by a Zealot, Ultra, Field Master or some other distinguished high ranking officer.
Background
The responsibilities of a Legion seem to vary. They are capable of performing tasks ranging from working as the bridge crew of Covenant warships to engaging in ground combat. [1] The violent nature of the Elite society likely demands that Legions be capable of engaging in varied forms of combat.
Warrior Crèche
A Warrior Crèche is likely a type of Legion where newer warriors hone their combat skills before being assigned to a regular legion. Warrior crèches seem to be specifically used for ground combat, and are carried aboard Covenant Fleets should any ground actions be needed. [2]