Halo: Spartan Assault

Halo: Spartan Assault takes place in the 26th century between the events of Halo 3 and Halo 4. Following the events of Halo 3, the human UNSC and alien Elites (Sangheili) signed a ceasefire to end a decades-long war. Gameplay follows through the perspective of the Spartan supersoldiers Sarah Palmer or Spartan Davis. Canonically, the events are played from the perspective of human military cadets, reliving the events inside a battle simulation.
Spartans Palmer and Davis are stationed on the planet Draetheus V when it comes under attack from a splinter group of the Covenant who have ignored the ceasefire. Palmer and Davis work to repel the invaders. This new Covenant faction led by Merg Vol discovers that Draetheus' moon is actually a weapon built by an ancient race known as the Forerunners. Merg Vol's Covenant activates the weapon, tearing apart Draetheus and triggering an evacuation. Spartan Davis gives his life to allow the remaining human forces enough time to escape. Spartan Palmer tracks down Merg Vol, kills him, sabotages the Forerunner weapon, and escapes the moon.
In the Operation Hydra campaign, Spartan Palmer returns to X50 in search of a mysterious signal. She discovers that the signal is actually a distress signal from Spartan Davis. Palmer reaches the core of X50, but finds Davis dead; the moon is using his remains to create an unknown device, which Palmer extracts for study.