Sharing this incase there's still others that don't know how to acquire the Legendary AR Eidolon Ally. First step is acquiring "Husk of the Pit" common AR that drop from Blades of Crota knights. If you don't have time to wait for the blades to spawn while on patrols, just load up the "Fist of Crota" story mission. Run past the battle and into the building, you'll see the ritual and a Blade of Crota will spawn, he has a chance to drop the AR. Sorry, not sure the exact % of drop, but I received my first one after about 14 tries, then another after 7 tries. If it doesn't drop, you'll need to quit the mission and try again. Now you'll need to purchase an Embalming Orb from Eris at the Tower (requires rank 3, and 10 black idols). Once you level up the "Cannibalism" Upgrade on the Husk of the Pit it will allow you to use the orb. Ta-da! Congratulations on your new Eidolon Ally! You will need this to upgrade to the Exotic AR Necrochasm once the hard mode for Crota's End is released. Hope this helped, happy holidays! :) edit: please like the topic to make it easier to find for others in the future^^