Kurama Clan's Kekkei Genkai
The Kurama clan (鞍馬一族, Kurama Ichizoku) is a clan of extremely skilled genjutsu users. This talent in genjutsu is due to the unnamed kekkei genkai that the clan possesses. However, once every few generations, a member of the clan will be born with such enormous skill in genjutsu that their illusions can actually produce tangible effects that occur in reality. 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 is a danger to everyone around them. They were originally one of the more prominent clans in Konohagakure, but the number of ninja in the clan has severely dwindled.