Colored by his past experience fighting the Supernaturals, Rex believed that he already knew every facet of their nature, especially their power and how they fight. But this conviction was swiftly debunked after observing the assembled army.
One observation is enough to tell him that he was gravely mistaken.
Rex relied on Mavenna to explain everything he was seeing.
Even if he had the System, the variety of this army would still leave him awe-struck.
It was completely different than the regular Supernaturals he fought.
He thought that at the very least, the surprise from the ancient times would come when the end phase of the First Breath was nearing—when much of the older Supernatural wakes up from their slumber already.
But he was wrong, even now, the deadliness of the ancient times can already be seen vividly.