https://www.youtube.com/watch?v=1ZNT-vqtVpI

It's a campaign mod that adds new weapons, enemies, extended levels/new areas and even a whole new level... Graphical update as well.. Made by a modding team that have been at this for a few years.. They've only released the first half of the campaign, still working on the 2nd.. But there's still quite a lot of new content in there that makes the experience completely new.. Played through it once on Legendary, now doing it on Noble which is their new highest difficulty... Very challenging but not gamebreaking.. If you enjoyed the original Halo, like classic style FPS... should check this out