I haven't read Glasslands so I don't know if it is explained there, but how did the Covenant get back together? Why are they against humans? And how was the Didact able to command them so easily?
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They are a split off faction called "Storm", they still believe in the Forerunner's as gods and in The Great Journey, a Sangheili named Jul Mdama created it from this Sangheili colony he ended up on in 2553. They went to Requiem to seek the Didact's help in destroying the Humans. [Edited on 11.06.2012 2:04 PM PST]