Savathûn's Ghost Immaru has approached the Vanguard with a dire bargain: the Witch Queen holds the information we need to pursue the Witness and will share it with us if we first contend with her sister, Xivu Arath.  Through the power of arcane rituals, the Guardians must gather tithes for Eris Morn so she can fulfill this dangerous pact with the Hive god of lies.