Halo 3: ODST is out now on PC as part of the Master Chief Collection. The spin-off of sorts doesn’t feature Master Chief, but from a view point as normal rookie soldier investigating the aftermath of New Mombasa.
In addition, the game includes Firefight, the co-op horde mode.
This leaves Halo 4 still coming, which will be the last game to be coming to PC as part of the Master Chief Collection. Soon all the Halo games (outside of Halo 5) will be playable on PC.
Halo 3: ODST is out now on PC via Steam (standalone purhase for RM12.50) and Microsoft Store. It’s also included in Xbox Game Pass for PC.