Nahir had no trouble casting offensive magic because his Fire magic had a destructive cleansing effect. The more his target had impurities in their body and mind, the more his flames' destructiveness would be unleashed.
Apart from the Eidums, the impurities inside the bodies of the inhabitants of Somuli were significant. For this reason, Nahir's cleansing flames could become very destructive. Since it also involved the mind, Eidums might also be affected because not all had pure hearts. Nevertheless, the destructiveness of his flames was far less compared to when someone with both impure body and mind was attacked with it.
One reason Nahir decided to give Kyran the benefit of the doubt was because he was unaffected by his flames. This spoke volumes about the younger man's impurities. But since Kyran admitted to being an outsider, as well as being the Dark Sage's successor, Nahir could no longer say for sure.
Anyway, that was unimportant at the moment.
