From Narutopedia
Every few generations a member of the Kurama clan will be born with an incredible talent for genjutsu. The clan member's talent in that field is such that their genjutsu is completely irresistible. Furthermore, the genjutsu is so powerful that it causes the brain to make anything that happens to the victim within the genjutsu physically real, allowing the clan member to potentially kill their opponents with genjutsu. Such frightening power is not without its drawbacks, however. The user can rarely control the full extent of their abilities, and as such their subconscious will often regulate that power, leading to the creation of a second personality in control of that power. This personality then overwhelms the original, turning the individual into a monster that's a danger to everyone around them.
