The Fallen King
The Magus is actually half-corrupted, as shown by the black mass that makes up the lower half of his body. Though he is resisting Ahriman's influence, he will occasionally become fully corrupted, taking on a black-red color scheme. This occurs when he is exposed to certain creatures. There are two varieties of these creatures: the spider-like type, which will only result in him changing color and needing occasional revival, and the flying type, which will carry him to another level and cause him to become hostile. The former can be healed by finding an elixir provided by the Ancestor, whilst the latter must be healed by following the Magus through the level he is taken to and defeating him in combat.
The Magus can also pass through the corrupted fog that is present on some levels, unlike the Prince. Situations such as this result in the player being required to control each character separately, and is the only time the Magus will be controlled directly. Oddly, despite the fact he seems to be able to fly behind the Prince during normal gameplay, the ability is lost during separate control, so the player must cope with only his magic abilities, as he possesses nothing in the way of acrobatic abilities such as jumping.
During the final sequence of the game, the Magus and his powers are 'combined' with the Prince, creating a character who has both the athletic ability of the Prince and the magic abilities of the Magus. This character also appears to be immune to most forms of the corruption, much like the Magus is immune to the corrupted fog.