Halo 4 was the first mainline Halo game developed by 343 Industries.
Halo 4 was released in 2012 and continues the story after Halo 3. It follows Master Chief and Cortana as they encounter the Forerunner world of Requiem and become involved in a conflict involving the Didact. The game introduced a new visual and narrative direction while retaining the series’ first-person combat foundation.
343 Industries was formed after Bungie’s departure from Microsoft to oversee future Halo development. Halo 4 was therefore the first numbered mainline entry made by the new studio rather than Bungie. Its launch began the Reclaimer Saga, a planned sequence of stories centered on humanity’s relationship with Forerunner technology.
Halo 5: Guardians and Halo Infinite followed later, so neither can be the first. Halo Wars 2 was also developed by Creative Assembly with 343 Industries, but it is a real-time strategy spin-off rather than the first 343-developed mainline sequel.