Hyūga Main Family's Juinjutsu

This seal is branded on all branch house members in the Hyūga clan by members of the main house. The main purpose is to seal the Byakugan at death, so that its secrets never fall into the hands of the enemy. When the Hyūga dies the seal disappears, sealing away the Byakugan with it. The secondary purpose is to control the branch house members, since it can be activated at will by a main house member with a hand seal only known by them to destroy the wearer's brain cells.

The seal is branded on a branch member's forehead when the next heir or heiress 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 concealed because of the seal.

Trivia

 * Because of the seal's resemblance to a swastika — which in actuality is a Hindu and Buddhist symbol but more commonly associated with the Nazi — it is replaced with an "X" in the anime version of the series to avoid confusion. 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. The Viz version of the manga has also taken care to always mention that the symbol is not a swastika whenever it is displayed.
 * 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.