Heretic is an ability which reduces dominion in the unit's province. Heretic can reduce both friendly and enemy dominion. The check for Heretic occurs directly after preaching and temple checks in the turn order sequence.

Each turn, units with Heretic may lower dominion. The chance of success of this check is a percentage equal to the Heretic ability value × 20; on a success, dominion in the province is reduced by one. Nothing happens if there is zero dominion in the province. Heretic values higher than 5 (i.e. with a chance of success higher than 100%) result in one guaranteed dominion loss per 5 Heretic value, with the remainder having a chance to reduce dominion according to the previous formula.¹⁾

Commanders with the insane or shattered soul abilities will sometimes engage in Heretical activities. This is actually a lesser form of regular preaching, but it is in favor of R'lyeh for the insane, or Ermor for the shattered.

what happens if no R'lyeh or Ermor in game? Where did this information come from?