From Halopedia, the Halo wiki
Unity is a game engine maintained by Unity Technologies. First released in 2005, it is a royalty-free third-party game engine available for use by any game developer, on a wide variety of platforms, in exchange for a subscription fee depending on the developer's selected payment tier.
Usage in Halo games
The mobile augmented reality game Halo 4: King of the Hill Fueled by Mountain Dew was created using the Unity engine, as confirmed by the presence of Unity files in the game's Android application package. It most likely used Unity 3.0, the latest version of the engine at the time of the game's development.
Games using the Unity engine
The following Halo games utilise the Unity Engine: