This now drops from the blades of Crota since the hot fix patch. It can be upgraded to the legendary auto rifle 'Eidolon ally' by Eris when you reach rank 3 with her and then to the nechrochasm in the hard version of Crota's end raid (January) I believe. I farmed mine from the fist of Crota story. Whizz past the screeching wizard into the building. First thing you come to is the blade of Crota ritual. Kill the blade and it may drop. Rinse repeat. I combined it with the kill hive from behind bounty and the kill hive with precision damage under a black wax idol bounty. Took about 20 runs to get the husk and the bounties so not too bad.