Halo wars is a real-time strategy (RTS) game that takes place in the Halo universe 20 years before the original game took place. Halo Wars is for the Xbox 360.
Throughout Halo Wars are locations with skulls that can grant special abilities once you complete specific tasks. These skulls are hidden on the maps, and once you complete the task for the skull, it will flash on the mini-map. The tasks consist of killing specific amount of certain creatures.