The Southern Schism

The Southern Schism, also called the Starry Schism, was a divide in the Faith of the Seven that saw two separate High Septons appointed by the Most Devout, causing a rift to form between the septs of King's Landing and Oldtown. It lasted for nearly three years, though the separate schools of thought persist across the continent to this day.

Dates and Succession
346 the new septon is crowned. Build up had occured for some seven years. Because the most devout nominated both of the original High Septons, it is as of yet unclear who is the heretic, and who the true heir to the title.