http://destiny.wikia.com/wiki/Crux_of_Crota
You need to have the eidolon equipped while killing crota?
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Straight BS I got the Crux (and so didn't my brother on a separate occasion) without even getting to lvl3 with Eris. Neither of our Husk's were upgraded to the Ally and my Bro's hadn't even been leveled at all! [b] You absolutely do not need a max Ally to get the Crux to drop[/b]