The Ecclesiarch is the leader of the Ecclesiarchy (a.k.a. the Adeptus Ministorum)
The Ecclesiarch is based in a fortress within the Ecclesiarchal Palace on Terra. Elected by the Holy Synod, the Ecclesiarch's main concerns are representing the Ministorum as a High Lord, the ordering of crusades, and monitoring the devotion of the Imperial citizenry. As the head of the Imperial Cult, he is an immensely powerful and influential figure.