1) Get the Husk of the Pit drop from Hive enemies or Nightfall
2) Read the perks of the Husk to see if it requires Thrall, Wizard or Knight kills, then go farm the enemy to get the Eidolon Ally rifle
3) Visit the Speaker and Eris at the Tower
4) Complete the 390 version of Crota's End, defeat Omnigul in The Will of Crota Strike
5) Use 'Essence of the Oversoul' item Eris gives you on the Eidolon Alley to get the Necrochasm