Archaon has the Mark of Khorne, Mark of Tzeentch, Mark of Nurgle and Mark of Slaanesh. This does not prevent Archaon from joining any unit that has a Mark of Chaos, nor does it prevent any other character that has a Mark of Chaos from joining Archaon's unit. In addition, Archaon may re-roll any Eye of the Gods roll, but the second result stands, even if it is worse.