Tengentsu

The Tengentsu (天眼通) is a special eye with the power to perceive anything and everything that is going on at that moment. The eye can also read people's thoughts and memories, as well as gauge their strength.

Acquisition
It is acquired by entering a cave within the Hino Country. Only someone with a very high level of magic power can acquire the Tengentsu's power, but gaining it comes at the cost of the individual permanently losing all their magic power.

Trivia

 * Tengentsū is the Japanese term for one of the abhijñā ("higher knowledge") in Buddhism. It is the "divine eye" that allows one to know others' karmic destinations.