This coin is granted once reaching a high point in the Crucible. It grants you a trip to Mercury where you have to win three times in a row, but if you lose all three you can NEVER (probably not) play it again. However each time you win it grants another try, the reward for winning is becoming and ELITE crucible Guardian. With some epic gear.
http://imgur.com/gFErBZU
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Wait, [b]MERCURY[/b]?