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You have to go on a long obnoxious Easter egg hunt first..
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Once you've completed the main story mission, you should get a bunch of quests from Saladin, Shiro-4 and Tyra Karn. One of these should be a quest called Patrol the Plaguelands. The text will say "[i]Complete the Anomaly Scan patrol near Archon's Keep.[/i]" This is the one that kicks off the Gjallarhorn quest.
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You have to do the ghost scan patrol mission in the archons keep. After finishing that saladin will give you the gjallarhorn quest
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You have to open/unlock the rise of iron book.