Was lucky enough to get my first one after only killing about 6-7 sword of crotas on fist of crota mission. Then I got my second on the second sword of crota I killed on the same mission doing the bounty, and my third just farming spin metal on earth and killed a sword of crota. I've only upgraded one to Ally cause it's not very good....but i want to see how the Necrocasm is eventually, so I maxed Ally as far as I could. I kept the other two Husks in hopes that Necrocasm will be great.