The most terrifying tribulation of the red dust tribulations was this one, the thousand lives heart-tribulation. It was an experience akin to being reborn a thousand times. If one's Dao-heart was weak, one would probably be lost forever…
After all, to Immortal cultivators, with the passage of countless lifetimes, one would begin to lose the memories of one's former lives. With so many personalities and experiences in one place…the original personality and person would be lost.
Experiencing a thousand lives, in and of itself, was enough to cause many Loose Immortals to be lost…at which point, the karmic sinflames would burn their souls to ashes.