"It is no mere curiosity that drew Calus to the lunar Pyramid. He is a consciousness that spreads out like a vascular system…" Eris says. "And he is attempting to become one with the lunar Pyramid, so that he may control it. So that he may wield its power."
It is now clear that Calus, at the behest of the Witness, is seeking to claim the power of the lunar Pyramid to aid the Witness in bringing about the second Collapse.
Eris is going to prepare a ritual of binding for Caiatl so that you may—together—attempt to sever Calus's final connection to the Pyramid before he completes his task.
For now, Eris urges you to return to the Leviathan and help contain the growing Nightmare there while there is still time.