I imagine Chief is either fighting the Forerunners or something like the Flood, but possibly more intelligent and harder to kill, pretty much if the Flood was equal to the locust in Gears, these new creatures would be like the Lambent. More evolved in a way, stronger, and can maybe even transform, and all forms would explode when they are killed. I think the world he is on is what someone thought it was, it has an inside and an outside. The outside would be more Urban and industrious, while the Inside is like Halo rings, broken into pieces, many of them too as there is a lot of room. They would feature terrain of every kind, including the Covenant's home planet.