[Wait until Merlin figures out that Watanuki's existence is a blind spot to Goetia and that these early conversations are going completely unnoticed. It's a fascinating trait that will become less useful in the future, when their enemies are aware of how difficult it is to perceive him and start deliberately looking for the blind spots he could be hiding in. But for now, it lets Watanuki learn things without giving away too much to the enemies of mankind.

As Merlin's look turns serious, so does Watanuki's. If the lazy wizard is being serious about something, then it definitely deserves care and consideration. And Watanuki has been learning to read between the lines. "No records of the calamity" doesn't mean it didn't happen, just that nobody survived to write the records down. And if Merlin himself, this powerful mage, is just an aide? Then yeah, this is going to be quite a battle. Watanuki is further motivated to learn as much as he can and become a mage capable of helping save the past.]

The fact that humans still life is proof of existence, isn't it? Or is this one of those 'if you travel back in time you can change the future' kind of things?