Mithrax tells you that he has studied the Vex and come to understand the drives that govern their behavior—which is why he finds their recent actions so confusing.
The Vex seem to be growing alongside your powers, acting to counter your growth as a Splicer. He feels as though there is something in their network no Splicer has encountered before. It is vital you continue to enter the Vex domain and collapse their networks.
He warns you powerful foes will emerge from the Vex domain and that, as a Splicer, you must be ready for them.
"Keep to the path," he tells you.