Hyūga Main Family's Juinjutsu

This seal is given to all branch house members in the Hyūga clan by members of the main house. Its main purpose is to seal their Byakugan ability when they die, preventing an enemy from learning its secrets. Its secondary purpose is to control the branch house, since it can be activated at will by a main house member with a hand seal that only they know about, to destroy the wearer's brain cells.

The seal is placed on the wearer's forehead when the next member of the main house turns three years old, (Neji Hyūga received his at the age of four on Hinata's third birthday), and is only removed upon their death. Branch house members tend to keep their foreheads covered because of the seal.

Trivia

 * Because of the seal's resemblance to a swastika (which is actually a Buddhist symbol), it is replaced with an X in the anime version of the series. It receives the same treatment in Viz's monthly distribution of the English manga, but the original design is retained in Viz's volume compilations.
 * Ao from Kirigakure killed a member of the Hyūga clan to obtain his Byakugan eye. Whether he killed a Main or Branch House member is still unknown, but it is however likely that he killed a member of the Main House or he removed the eye of a Branch member while his opponent was still alive; whether the Hyūga seal prevents this is unknown.