A Ruling King is a concept brought up in the 36 Lessons of Vivec which seemingly refers to an individual who has become a complete master of the world with no equal. The first known Ruling King was Vivec, followed by Tiber Septim, and then Nerevar.
According to Vivec, in order to become a Ruling King one must "suffer much that cannot be suffered" and "weigh matters that no astrolabe or compass can measure." An individual cannot be a Ruling King if they consider any other person equal in stature to themselves as a person who sees another as their equal rules nothing. However, each Ruling King has a double that they must overcome who is identical in a manner that if they were to switch roles, they would not know. This occurrence is associated with the number "11" in Vivec's Lessons, which is the Number of the Master.
A Ruling King is redeemed by each action they take, and are drawn back to the world upon death. It is stated by Vivec that with the rising of the next Ruling King, Vivec will confront them and kill them over and over again so that they can learn from his punishment and achieve his form, as the Ruling King must remove its maker. It is implied that CHIM may be required to accomplish this, as the Ruling King must "reach heaven by violence." Vivec is believed to have used this obscure teaching technique with Tiber Septim.[OOG 1]