I think the general consensus is that Lavos was not summoned, merely predicted. And, truly, I think the Reptite technology was easily advanced enough to perform such a feat. What's their technology level, anyway? Enough to build a fortress, that's for certain. Crossing that with our own history yields... well, at least a mid-second millennium BC, I'd guess. About the same level of achievement as the old Babylonians. Now, the Babylonians were master star-watchers and astologers, and were able to predict cosmic events with great accuracy. There is a story that a Greek sage, Thales of Miletus, predicted an eclipse would occur during a battle between the Ionians and the Persians... but, in fact, it was the Persians who had the great astrologers. Likely it was one of their number who performed the feat, and not Thales. The point is, the ancient star-watchers were very, very good at keeping records, and predicting the heavens (there are cuneiform inscriptions with what must be the results of thirty years of star-watching!) They were so influencial, we still use their system for some geometry (the 360 degrees to a circles is the old Babylonian system.) Anyway, the Babylonians were capable of incredibly accurate predictions. I'd wager the Reptites were similar.
There is another simpler explanation, however. New stars, supernovae, and comets were harbiners of sorts in ancient cultures... often ill-omens. A red star would be no different. It might just be the most basic of predictions, based more on religion than on science. A new star appears, that is not good - the heavens have changed! The star is red - the colour of blood, of war, and death. Again, not good. What do you think someone who knows the sky in the ancient world would interpret of such a portent? I think it's just that simple. No scientific prediction. No magical summoning. Just a bad-omen that turned out to be true.
