Mara's Corsairs have informed her of your encounter with the Wrathborn on Europa. As she herself has been monitoring Xivu Arath's movements within the Ascendant Plane, she's reached out to share what she knows.
"While I've yet to discern an exact stratagem," she tells you, "they may be trying to wrest control of the Warmind's destructive power on the Witness's behalf."
She then directs you to the Seraph bunker on the Moon, where her scouts report that the Hive are preparing an assault.