Kekkei Genkai

are abilities passed down genetically in specific clans. Kekkei genkai and their related jutsu can't be taught or copied by others, but it is possible to confer the ability to another person if it's localized in a particular organ like the eyes via organ transplant, like Obito passing his Sharingan eye to Kakashi. Kekkei genkai abilities that work via the user's eye are called dojutsu.

Kekkei genkai should not be confused with hiden jutsu. Hiden jutsu are exclusive to a certain person or clan, but only because they have kept others from learning how to perform those jutsu.

Kurama Clan's Genjutsu
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, so much so that only knocking a person out cold has any chance of awakening them from it. 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.