In Halo 4 the "composer" was an extremely powerful energy beam, if memory serves.

It is implied that the "composer" was in fact the original terraformer of planet Earth; the Didact is an enemy in the game because it is attempting to push the "reset" button when the civilization is developing healthily.

To be honest I didn't even play the game much... was just fascinated by the concepts. It was my brother who played the game fully.