He counts as having the Marks of all the Chaos gods for the purposes of choosing his army (ie, you can include units with any of the Marks, or any Daemonic troop , etc). ln addition, he has the following special rules: Magic Resistance (2); Level 2 Tzeentch Sorcerer; Immune to Psychology. Attacks, spell effects and anything that does not affect or has a reduced effect against followers of Nurgle (for example, the Skaven Plague Censer) also have no effect or reduced effect on Archaon.