Is Halo chess a mod?

Since halo chess is neither included with the game nor possible to create with the game, is it a mod?

Well yes, precisefly what halofanboy12 said. It was written in C on unipaas, and I was fortunately able to help develop it. It is a new gametype, as you can't obviously just make a 'chess' game out of slayer or CTF. It is a mod as you must download it from file shares and it is not shipped.