The Miasma
The violet flames didn't only distort the pentagram and start devouring it, they also allowed Iodel and Ulon both to sense the dark energy from the pentagram.
Even though well-made and seemingly powerful, the pentagram in the center of the town was nothing but decoration, it was a trick from the enemy, a mere distraction that would take all their attention away while they were doing something with what was the true centerpiece of the entire plan, the real large pentagram that was hidden by an array.
Nobody could sense or see the pentagram, and even after Ulon had attacked with all of his might, the array that was hiding the pentagram wasn't gone. It flickered for a short time, for a couple of seconds, which was how Iodel, Ulon, and the rest were able to see the two figures and notice the real pentagram, but that was all that he managed to do.