Archdemon is one of the Old Gods manifested in a form of powerful dragon, tainted by darkspawn. It is far more intelligent than an average dragon and it is purely evil creature. Under the surface darkspawn search for the sleeping Old Gods and when they find on of ancient dragons, it is immediately afficted by the taint.
Paarthurnax is a powerful dragon which appears in a game The Elder Scrolls V: Skyrim as the leader of the Greybeards, an ancient and honored order. He’s an ancient and immortal dragon and there are a lot of signs of won battles on his body like tattered skin or broken horns.