From Halopedia, the Halo wiki
Operation Trebuchet (OPTRE) is a fan-made total conversion mod for the military simulation FPS Arma 3. Developed by Article 2 Studios, the mod is an attempt to transplant the Halo universe into that of a more realistic military simulation. The mod is set during Operation: TREBUCHET, a massive-scale counterinsurgency operation conducted by the UNSC during the Insurrection. Players can play as UNSC Marines, UNSC Army, naval officers, SPARTAN-IIs and Insurrectionists.
OPTRE originally started as a different Arma 3 mod known as The Eridanus Insurrection, intended with the same goal of bringing Halo to a more realistic setting. In 2015, The Eridanus Insurrection ceased development and work on OPTRE began.. Since 2015, the development team for OPTRE has created a multitude of assets from across the Halo universe.
First Contact is a side mod for OPTRE, set during the earliest days of the Human-Covenant War. The mod introduces playable Covenant to the game, alongside other war-era equipment such as the Halo 3-styled Marine armor and Halo: Combat Evolved Anniversary-styled MJOLNIR Mark V.