Serialized in Japan in: Shogakukan's Shōnen Sunday
Story(source:wikipedia):Five hundred years ago, a feudal lord was attacked by demons wanting to retrieve his mysterious inner power, the invincible power that can help them take over the human world. The famous mage, Tokimori Hazama, was called upon, and with his 2 students, managed to save the whole land. But they fell ill, the feudal lord was killed, his power was also buried with him at a site called Karasumori.
In the present day, Yoshimori Sumimura and Tokine Yukimura, heirs of the Hazama clan, became the kekkaishi protecting the land and the power. They use a technique called Kekkai, a form of magical barrier, used to capture and destroy Ayakashi (monsters/demons) that are drawn to this land. Any demons that come near the power will become stronger and therefore Yoshimori and Tokine are to guard the land from intrusion of the Ayakashi. To do that, he first needs to know what the power actually is, and also strengthen his poor kekkai skills.