Forge volunteers to destroy the sun and make a supernova and tells the spartans that he need to them the fight. they use the ship The Spirit of Fire to slingshot around the sun and escape just as the Forerunner fleet and Forge are obliterated by the planet's blast. Without its reactor, the The Spirit of Fire is left drifting in space. The crew goes into cryonic sleep and Cutter takes a last look at Forge's empty cryo tube.
If you beat the game on Legendary mode, after the credits Serina wakes Cutter and tells him that something has happened