After wave 15 in Campaign mode, Vez'nan will summon the Archdemon (which he affectionally calls an 'old friend'). In response, King Denas will order one of his men to drink a serum, transforming him into a Paragon. Both the Archdemon and Paragon will then proceed to slowly move toward each other, and a progress bar will appear at the top of the screen showing the relative positions of the Archdemon and Paragon.
The Archdemon ignores all normal enemies and will only block the Paragons. It has a special slam attack which deals 2200 physical area damage and stuns victims for 1 second (damage area: 180), and will interrupt the Paragon's own slam attack if it is being channelled; this attack is manually activated by the player by tapping on Vez'nan when it is ready. Even though it is a melee unit, its basic attacks deal magic damage.
Once the Archdemon enters the castle or when three Paragons have been defeated (whichever happens first), the player will win the stage, while letting the Archdemon die results in an automatic defeat.