- MLG Cheehwawa
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- Fabled Mythic Member
ODST is a prequal to Halo 3, occuring while the In Amber Clad is in slipspace on its way to Delta Halo in Halo 2. You won't see the Chief in it.
REACH is a pequal to Halo 1, occuring about a month before. If Spartan-259 and -320 are playable characters, it is unlikely that we will play as the Chief (though we will probably see him), Fred, Kelly, etc., as -259 and -320 were not with Alpha Team, and they were not inside of CASTLE base either. They also did not leave with the Chief when he returned to Reach with Johnson before Halo 2. My guess is we will play as 5 Spartans from Team Gamma, the 11 unnamed, unnumbered Spartans left by Fred to guard the orbital defense facility while the other Spartans went their separate ways. This would open us up to some new stories, offering ner perspectives on the battle of Reach.
If you want to learn more about the destruction of planet Reach, read Halo: First Strike.