I farmed the mission "The Fist of Crota" on earth. Once you begin the mission just fly through the enimies until you reach the acolytes crouching in a circle, the text "The minions of Crota have started a ritual" will appear on screen. Proceed to kill the Blade of Crota that spawns. If you do not recieve the drop, return to orbit and repeat steps 1-2. It worked for me after about 30-ish miniutes of farming. Also if you want to gain some Crota's Bane (Eris Morn) rep. on the side, be sure to pick up any relevant bounties I.E. "Kill 10 hive majors or ultras", or "Hunt the knights who serve as blades of Crota", and so on. That's about it, I hope this helps!