"Phylaks uses her soldiers like pieces on a game board. You must draw her out. Give her no choice." —Variks the Loyal
Ghost picked up some intel from the Fallen troops you defeated. Phylaks has platoons patrolling Asterion Abyss, trying to claim Vex-held territory for Eramis.
Defeating these platoons will draw out Phylaks's commander, Elenaks. This should lead to more information about her whereabouts.