At first Ba'al was relatively dismissive of Saber's answer, as one might expect from a being as old as him, he had his own opinions on 'magic', namely that it didn't exist. But perhaps he was cynical from playing 'God' for so long, and having so much of what he did interpreted as magic by primitives. So he would hold off definitive judgement until he got proof one way or another, which seemed unlikely in this situation.

Then, the impressive bit, as Saber released some force, her sword shimmering into existence where there had been nothing before, gleaming even in the dark. It was a deadly weapon if he had seen one. He only wondered how it would match up against his Shikra Blade, and wished he had it here. 

The announcement of identity was taken...unusually, perhaps a lot more calmly than she might expect, Ba'al folding his arms and giving her a bemused smirk, obviously finding something funny about this situation. "Well now, I didn't expect to find myself with royalty, especially one so fabled." There was a slight bow, his concession to one technically in equal of status to he. "I've been to Camelot, and I would cede that it is a nice enough place, though I have a feeling that my Camelot may be different than yours. Still, it is a pleasure to meet in person the Once and Future King." Him actually being female was something he could deal with, given the fact that Merlin was actually an Ascended being in his universe, so gender was a small issue. Especially again when it came to being an asexual parasite.