Before leaving Nasir Town, Mr. Byrne first checked the barrier spell protecting the town.
Viscount Bast had once given him a bag of black stones, which were actually "barrier markers" created through alchemy.
He had buried the barrier markers around the town, and by chanting the spells inscribed on the stones, he could activate them.
Once activated, they would generate a barrier spell known as "Dark Mirror," which would continuously consume the magic power inherent in the stones while manifesting strange black mirrors in various selectable locations within the barrier's range.
In principle, it was a simple transport array that carried the power of a curse.
The activator could allow certain individuals to freely enter and leave through the Black Mirror, transporting them to different locations around the town, while the enemies within the barrier range would be persistently suppressed and weakened by the curse power of the Black Mirror.