Satsugan

The Satsugan (Eye of Insight), is a dojutsu kekkei genkai, that members of the Kamiya Clan possess. The Dojutsu originated hundreds of year ago when a faction of the Uchiha had a falling out with the main branch of the family and broke off, leaving the land of fire. They wondered east into the islands until settling in the main land. It was a quieter place back then, much less populated than the fertile lands and not as accessible to as the other lands. While settling in the island nation the self exhiled Uchiha joined up with a clan called the Sado, creating the Kamiya clan, and after centuries of a thinning genetic pool and random mutation the Sharingan of the Uchiha faded into the Satsugan of the Kamiya.

Abilities
While the Sharingan's abilities consist of two parts: the "Eye of Insight" and the "Eye of Hypnotism" the Satsugan focuses mainly on the Eye of Insight, and lends very little to the eye of hypnotism.

Like its distant cousin the first of the eye's powers is being able to see chakra flow. The eye itself gives colour to chakra, allowing the wielder to differentiate them. They are also able to tell if a person is under a genjutsu because the person's chakra flow would be irregular. Through training and mutation this ability has been heightened to beyond that of the Sharingan, being able to see chakra left by people's foot prints and rubbed off onto surfaces, making it ideal for tracking. The Satsugan can also see chakra through solid surfaces for a limited distance, ranging between 50 and 250 meters with a limited telescopic view depending on the user.

The Satsugan's second-most prominent ability grants the user an incredible clarity of perception, allowing them to easily recognise genjutsu and different forms of chakra, though not to the same extent as the Byakugan. This also allows the user to pick up on subtle details, enabling them to read lip movements or mimic something like pencil movements. As the Satsugan evolves, gaining more tomoe, this ability extends to being able to track fast-moving objects before finally giving some amount of predictive capabilities to the user, allowing them to see the image of an attacker's next move from the slightest muscle tension in their body and counter-attack or dodge without any wasted movement. However, even though the user may be able to see an attack or know it is coming, their body may not always have time to react.

Because of the thinning of the genetic pool of the Kamiya compared to the Uchiha, the Satsugan has lost almost all of its ability to cast genjutsu, it does offer a high resistance to all but the most powerful of genjutsu.

Although the Satsugan consumes the user's chakra while active, the amount of chakra that the user must exert is so insignificant that they can keep it active constantly.

Receiving the Satsugan
The number of clan members who have possessed the Satsugan seems to be ever declining as the world gets more peaceful, and skyrockets up during times of war. The reason for this is part of the mutations that caused the Sharingan to become the Satsugan was that the hormones that develop a Kamiya's eye into the Satsugan need to be much more potent than those of a Sharingan, requiring a more traumatizing moment in the person's life to awaken it. The requirements for the release of such a hormone are someone very close to them dies, becomes traumatized or has a near death experience. There was a time briefly in their history where these moments were simulated using powerful genjutsu, but it was decided to be too harsh and traumatizing to the people they were doing it to, and were subsequently stopped.

Once the user has awakened the Satsugan they can freely activate it and deactivate it by will. Also like the Sharingan it seems to glow in the dark. It is believed that the strong sense of loss is required to awaken the Satsugan, and its journey continues until the wielder either dies, or comes to a strong resolution in life.

Stages
Once the wielder awakens the Satsugan, they begin with one tomoe, taking up the entire pupil of the eye. The wielder of the Satsugan then, through continued use and training, changes its form further, which involves the eye maturing fully until it has three tomoe. As the Satsugan develops, the user gains heightened abilities through the heightened ocular sense. For example, they can at least see chakra with one tomoe, predict a person's movement and resist some genjutsu with two and completely use the Satsugan's abilities when it is fully matured.