....This is why I'm glad that Square-Enix operates under Japanese law when it comes to fan-work (Japanese companies pretty much are hands-off in regards to fan work, and will actually publish some of the better stuff...)
...As for what Hunter Anubis is saying, he's on the right track: If you re-named the mod and the units, and as long as the models used are original 3rd-party models (and especially if they're slightly changed), then Bungie can't do squat: The reason is that now it is, for the purposes of copyright law, a parody of Halo, and not Halo itself, and parodies are protected by copyright law (Wierd Al Yankovich does not HAVE to get the original artist's permission when he records a parody of a popular song, but he does so as a matter of courtesey).