Covenant

From ILB

The Covenant are not a single race of aliens, but a coalition of aliens lead by two dominant races - the Prophets (http://library.psyjnir.net/?id=49) and the Elite (http://library.psyjnir.net/?id=44). Commanding the religious and military aspects respectively, they have swept the galaxy and enslaved other races within their thrall. They attacked Harvest (http://library.psyjnir.net/?id=123) without warning and began destroying colonies by glassing them. They seem to have no interest in enslaving mankind, only finding Earth and eradicating them. Most of their broadcasts show a religious zeal and they seem to consider humans an affront to their religion. They have been largely successful due to their large numbers, fanaticism and powerful plasma based weapons.

More on the Covenant via Halo Library (http://library.psyjnir.net/?id=2) and The Halo Story Page (http://halosm.bungie.org/story/covenant.html).