This clip, seemingly done for Spartan Ops, was done by Planet Jump animation studio for the launch of Halo 4 last year. Part-animated clip, part-motion comic, its got an anime style and helps explain, at least partially, why the Covenant are hanging around the game. Better than the game manages, anyway.
You can see the video itself here, while there's a big collection of Alezander Watt and Antinori's art below.
Halo 4 Promotional Film [Planet Jump]