Units with the Heretic trait have a 20% * heretic level chance each turn to reduce the dominion strength in the province they are in by 1. A common way to give any unit the Heretic trait is the Stone Idol item.

Example: A Heretic(3) in a province has a 60% chance to reduce dominion strength by 1. With three Heretic(1) commanders in a province there, are three 20%-chances of the dominion strength being reduced.

A heretic in a province with 0 dominion strength has no effect.