At the heart of Archaon's army is his old warband, the Swords of Chaos. Only the strongest have survived in his long quest, leaving a cadre of hardened troops.
Source: Warhammer Fantasy: 7th Edition
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Archon's army must include at least one unit of Knights of Chaos. Archaon is the only character allowed to join this unit (though he does not have to). In addition, if Archaon leads this unit, then it becomes Immune to Psychology.