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: 8 - Words: 32,312 - Reviews: 40 - Favs: 13 - Follows: 21 - Updated: 3-27-13 - Published: 1-20-13 - Eldar Master Chief/John-117