 Four years later , the Chief and Cortana awake to find they are being attacked by a Covenant splinter group ( Halo 4 ) . Caught in an artificial gravity well , they are pulled into the artificial Forerunner planet of Requiem , along with the UNSC ship Infinity . Master Chief , fighting Covenant and AI soldiers called Prometheans , unwittingly releases a hostile Forerunner warrior called the Didact . While seeking a way to combat the Didact , Cortana reveals that she is becoming rampant , a state in AIs when their knowledge base becomes too large and they gradually go insane . Master Chief is eventually contacted by a Forerunner consciousness called the Librarian ( The Didact 's Wife ) , who tells him about the Didact : during the crisis with the Flood , the Didact tried to use a device called the Composer to transform humanity into creatures like the Prometheans . He did this because he needed extra soldiers in the war . His own race then imprisoned him along with Forerunner soldiers and humans he had already converted . The Composer was hidden . After the Didact leaves Requiem and retrieves the Composer , Master Chief pursues him to Earth , with Master Chief and Cortana working together to destroy the Composer . Master Chief succeeds , but Cortana sacrifices herself to save him from the resultant explosion . Master Chief is later found floating among the rubble , and taken back to Infinity . 
