At Fycro's level, certain powerful skills could be seen at a glance. As long as a skill was below grade-13, its numerous flaws could be identified. If such a skill was experienced once, it was easier to detect more flaws.
Strictly speaking, Endless Night was not considered a single skill. It could even be considered a magic array. To see through the magic array, you had to be outside of it. This was impossible to do from deep inside.
This was the case for mages, too. Divine Protection was a divine rune charm array with comparable power. Hence, if mages wanted to break through it, they would have to exert tremendous effort. If Saleen, however, spotted a single spell, Fycro worried it would lose its effect.
