Kyatsu

Kyatsu, was a Kagegakure Dark Lord of Ashi, and a master of Telekinesis. Obsessed with eternal life, Kyatsu experimented with ways to cheat death and create new life. He played a large role in Ashi events, inciting Kazemaru, whom he trained in the ways of the ninja and the Ashi. Kazemaru, concerned with being replaced as Kyatsu's apprentice, eventually killed him in his sleep.

Training Kazemaru
""Tell me what you regard as your greatest strength, so I will know how best to undermine you; tell me of your greatest fear, so I will know which I must force you to face; tell me what you cherish most, so I will know what to take from you; and tell me what you crave, so that I might deny you."- Dark Lord Kyatsu to Kazemaru"

Kazemaru learned from Kyatsu over the course of decades, during which Kyatsu taught his apprentice everything he knew to prevent the power he had amassed from being lost forever. During Kazemaru's training, Kyatsu gave his apprentice access to his Ashi scrolls. Kyatsu's lessons involved forcing Kazemaru to face his fears, denying him pleasures, and taking from him the things he loved. He taught Kazemaru that emotions such as envy and hatred, though necessary to master, were simply means to the end of casting aside usual notions of morality for a greater goal. Kyatsu also lectured his apprentice on the means of taking power, with the eventual goal of control over the 6 Great Nations.

Kazemaru, meanwhile, began training his own apprentice, Ryun Uchiha, without Kyatsu's knowledge, and may even have trained other apprentices before Ryun. Kazemaru would not kill his master, however, until he understood the lessons of Kyatsu and had become powerful enough to defeat him.