First Contact will be developed on a slower timeline than the main Operation: Trebuchet mod, as it is being considered a side project by the team. First Contact looks to add Covenant troops to its already impressive Arma 3 mod.
Article 2 Studios is the indie development group behind Operation: Trebuchet, and are also working on an expansion called First Contact.
The mod is one of several attempting to bring Halo to Arma 3, although it appears to have the most development time behind it.