After being burnt by the Illusory Soul Fire, the victim's soul would be ignited. If they had no distracting thoughts and their mind was clear, they wouldn't feel the burning at all.
Those with staunch resolve wouldn't be injured too much as long as they kept a firm mind.
On the contrary, if those with weak resolve had so much as a distracting thought or negative mood, those ideas would ignite and feel extremely painful, as if their soul was on fire.
In the future, many people would take the initiative to receive the burning of Illusory Soul Fire in order to temper themselves, and when powerhouses chose disciples, testing them with this was a necessary step.
But, this kind of tempering had to be done in moderation. If it was too excessive, one's soul would truly burn, which would be ruinous.
Lin Yun hadn't expected to see this thing in this era, let alone disguised as the flames of the Undead.
