Heizenmon Incident (平禅門の乱)
TAIRA no Yoritsuna, who was the steward of the Hojo Tokuso family (main branch of the Hojo family), avenged a senior vassal Yasumori ADACHI and other vassals of Yasumori's faction, with whom he had conflicts in political lines, in the Shimotsuki Incident in 1285, which was a year after Tokimune HOJO, who was one of the regents of the Kamakura bakufu (Japanese feudal government headed by a shogun), died and Tokimune's son Sadatoki HOJO became the ninth regent. For a while afterwards, Yoritsuna carried out politics focusing on procedures, for example, frequent submission of additional laws, but in 1287, at the time when Seii Taishogun (literally, "great general who subdues the barbarians") MINAMOTO no Koreyasu was given the title of Imperial Prince and became Imperial Prince Koreyasu, Yoritsuna changed his attitude toward politics suddenly and worked toward a reign of terror.
With Yoritsuna's support, Sadatoki established the tyranny of the patrimonial head of the main branch of the Hojo clan in which Sadatoki himself was the leader, but he came to be disturbed by Yoritsuna's reins of power and, finally in 1293, taking advantage of the turmoil caused by the Kamakura Earthquake, Sadatoki invaded Yoritsuna's house in Gyojigayatsu in Kamakura and made him commit suicide with his sword. It has been said that this incident was caused by slander made by a legitimate son TAIRA no Munetsuna, who had not got along with Yoritsuna.
After that, the momentum of miuchibito (private vassals of the Tokuso) including the family of Yoritsuna was diminished at one point, and Sadatoki started his tyranny. Vigorous vassals such as Akitoki HOJO and the Adachi clan who were forced to give in the Shimotsuki Incident gradually came back to the center of the bakufu.