I know this has been done before, but nevertheless, here is a Halo/Warhammer 40k Fusion Fic. The Covenant pledge themselves to the Four Powers, and humanity and the Eldar enter a tense and uneasy alliance to stop them, waging war throughout the galaxy. Rated T for reasons I think are easy enough to figure out. May be rated M later.
Rated: T - English - Sci-Fi/Adventure - Chapters: 10 - Words: 41,176 - Reviews: 56 - Favs: 28 - Follows: 35 - Updated: 8/29/2013 - Published: 1/20/2013 - Daemons, Eldar, Master Chief/John-117, OC