Hyūga Main Family's Juinjutsu

This cursed seal is used by the Hyūga clan in order to protect their kekkei genkai, the Byakugan. Members of the clan's main house brand members of the clan's branch houses with the cursed seal so that, when they die, their Byakugan will be sealed away, preventing its secrets from being learned by enemies. The cursed seal can also be used by the main house to discipline branch house members: if a main house member uses a special hand seal, the branch house member will experience intense pain, discouraging them from disobeying the main house again. If the pain is great enough, it can even kill the branch house member, destroying their brain cells.

Members of the branch house are branded with the cursed seal on the third birthday of the clan's next heir or heiress, regardless of their own age at the time: Neji received his when he was only four years old, when Hinata turned three. Cursed seals are placed on the recipient's forehead, for which reason they tend to keep it covered, such as with their forehead protector. Neji for years struggles with the idea that his cursed seal binds him to a life of servitude to the main house, likening his fate to that of a caged bird; because the cursed seal only disappears at the time of death, dying would be the only way he could be free. Neji's father, Hizashi, struggled with the same view, but took comfort in the fact that he could choose the manner of his own death, the most freeing and significant choice possible.

Trivia

 * The cursed seal's design (卍) is based on the manji, a Hindu and Buddhist symbol. Because the manji's inverted form (卐), the swastika, is commonly associated with Nazism, the anime instead uses an "X" for the cursed seal, to avoid confusion. Viz's monthly distribution of the English manga makes the same alteration, but their volume compilations use the original design. Whenever the original design is shown, Viz makes sure to explain that it is not a swastika.
 * Ao from Kirigakure killed a member of the Hyūga clan to obtain a single Byakugan. It's unclear how he was able to avoid the effects of the juinjutsu, whether his victim was a main house member, or the Byakugan being removed from a branch house member before death, whether the cursed seal prevents this is unknown. Like the Hyūga, Ao takes steps to protect his Byakugan from theft.