The "Sins of the Prophets" open beta was released last Friday but pulled after some serious issues were discovered. The "Halo" based mod for the PC 4X sim, "Sins of a Solar Empire" returned Monday though and is ready for public consumption once again.
"Sins of the Prophets" is a total conversion mod for "Sins of a Solar Empire" with ship models based on "Halo" lore, a refreshed user interface, new technology research trees, custom maps, new musical score and voice overs. The gameplay has been tweaked as well to make battles more fast-paced along with the size of planetary gravity wells.
The initial beta release only features UNSC ships but the all-volunteer team behind "Sins of the Prophets" is currently working to add the Covenant as well. The Flood are included in the beta release though as the alien parasite takes the place of the pirate faction. Eventually, a Forerunner faction will be added as well.
You can download the mod directly from the "Sins of the Prophets" site on ModDb.com. Note that this is a beta so don't be surprised if you run into any crashes or bugs. In fact, you can help the team behind the mod by posting issues to the forums at SinsOfTheProphets.com if you do run across bugs.
"Sins of the Prophets" requires a copy of "Sins of a Solar Empire" with the "Entrenchment" expansion. The best bet is to probably buy "Sins of a Solar Empire: Rebellion" from Steam as "Prophets" will eventually upgrade towards it and it contains the game plus all previous expansions.