Ephialtes, with Pericles' party behind him, takes most of the old Council of the Areopagus's powers away. He is murdered soon after.
Not only was the Council robbed of most of its jurisdiction by Ephialtes, but Cimon also, on the charge of being a lover of Sparta and a hater of the people, was ostracized
Many modern historians consider these reforms the beginning of the radical democracy Athens would become famous for. After Ephialtes' death, leadership passed to his deputy, Pericles.
Based on Wikipedia: Ephialtes
In the late 460s BC, he oversaw reforms that diminished the power of the Areopagus, a traditional bastion of conservatism, and which are considered by many modern historians to mark the beginning of the radical democracy for which Athens would become famous.
In the wake of his death, the political leadership of Athens passed to his deputy, Pericles.