Tenkigan

The is the very influential and fundamental dōjutsu used by the Raika Clan. While it is unknown who first bore the eye, it can be assumed that this ancestor of the dojutsu was also the ancestor of the Raika Clan. Known to be exceptionally rare, there are no current inheritors of this legendary dōjutsu, though that may soon change with the current crop of promising Raika members. The eye itself shares little to no genetics with the Byakugan, Rinnegan or Sharingan, leading many to believe that it may have existed before the Ōtsutsuki arrived on Earth, making it a dōjutsu that could very well be older than the world it inhabits.

Background
As previously stated, the eye itself is very old, having existed long before the current timeline, and is rumored to have been in existence before the Ōtsutsuki clan arrived. The itself is very powerful and influential, and is thought to be the reason for the Raika's existence, with a bearer of the eye organizing several members of society into a clan that he led.

The eye itself appears as a storm gray eye, with traces of silver in the center as well as a silver pupil that is almost non-discernible in its appearance.