SPOILERS AHEAD

         
         This is what is being said about halo 3 endings all are confirmed 
         except the halo 4 plot but hey its a real neat idea.
          
          Completing campaign on legendary is definitely worth the pain. Not
          only do you get achievements but you also get an exclusive ending
          at the end, after the credits. A cutscene will play. After that 
          cutscene, another one will play. A may be a mere 5 seconds long but
          it says it tells a lot if you know what's going on.

          HELLO HALO 4! You pretty only much know what's going on unless you 
          have read the books, OR have a magazine that summarizes everything 
          for you, YES!

          *SPOILER WARNINGS!*
          What happens is Master Chied's half-ridden ship is floating towards
          a black planet.  
 
          The black planet's name is Onyx. And yes, there are other Spartans,
          not just John, or Master Chief. Their names are Fred, Kelly and 
          Linda. They're all trapped on Onyx - insied a "fallout shelter" of
          sorts that's designed to protect its occupants in the event of the 
          Halos being activated - but they're alive. 

          Also, Cortana is not the only learning A.I. construct. She and 
          others were created by the UNSC's Dr. Catherine Halsey, (who dies 
          in Halo 3) who is traped with the remaining Spartans on Onyx. 
          Dr. Halsey made Cortana in her own image. Not too hard to notice...
           
          PEACE LOVE AND GUMMI BEARS FOR ALL:: NOZGOTH-076