Complex sentence: Spartan Assault is set between the time of "Halo 3" and "Halo 4" and the game will launch with 25 single-player missions, with more content expected to be released later.
Split into simpler sentences: Spartan Assault is set between the events of "Halo 3" and "Halo 4". The game will launch with 25 single-player missions, with more content expected to be released later.