Elio observed the sky where the monster had disappeared.
From the implication of what it said at the end, it still didn't know about the tunnel beneath the cores.
If in all this time it hadn't found Zara and the others, it would probably just watch from the heights.
"Selene," he turned to her, "I need you to send summons discreetly to warn the builders. They must not leave the tunnel under any circumstances until I arrive."
Lucien noticed the tension in Elio's voice. "We need to talk privately," he began, but Selene interrupted.
"But the barrier… If it fails we are done," she said. "It should have failed already today according to the normal pattern."