http://destiny.wikia.com/wiki/Crux_of_Crota
You need to have the eidolon equipped while killing crota?
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Nope, First time had a maxed alley in the Vault, Second had a Necro on character (not equipped) Third had a unleveled alley on character. Had glowho equipped first time but not second and third. It's RNG like everything else.