Peter the Great had his son tortured to death, changed the law so he could pick any heir, and died without naming one

Tsarevich Alexei, Peter the Great's son by his first wife, opposed his father's reforms and fled to Austrian protection in 1716. Peter's agents persuaded him to return in 1718 with a promise of pardon. Instead he was made to renounce the throne and was interrogated under torture. A court of senior officials sentenced him to death. He died in the Peter and Paul Fortress on 26 June 1718, two days after a final flogging^†. In 1722 Peter issued a statute giving the emperor the right to name any successor. When he died in February 1725 after an illness, he had not named one. A story says he wrote "Give all to..." and could not finish^†. Prince Menshikov and the guards regiments put Peter's widow on the throne as Catherine I, a former servant from Livonia. The 1722 statute produced 70 years of palace coups and five empresses. Paul I replaced it in 1797 with male primogeniture, which held until 1917.