http://destiny.wikia.com/wiki/Crux_of_Crota
You need to have the eidolon equipped while killing crota?
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I got it last week. The only requirement is that you are part of a fireteam that kills Crota on Hard difficulty. You do NOT need need the husk of the pit/eidolon ally. I know folks who have multiple crux but not a husk of the pit. You don't need to be wearing Glowhoo.