If a Prince of Light and a Heir of Doom started fighting each other, who would win and how?
The Prince of Light, no contest.
Princes are a combat class. Sometimes that combat is metaphorical, but they are always fighting and they are very good at it. Light isn't a combat aspect, but it is involved with strategy and if one hero has much more light than the other, they are highly favored.
Heirs are not a combat class. They tend to go with the flow, and can be quite defensive, but are generally about buffs. Doom is a combat aspect, but not for heirs. The definition of an heir of doom is approximately "always looses battles"
The battle would play out differently depending on how far actualized each is, but the prince would almost invariably win regardless of how far the session has gone.
The Prince of Light starts the battle by using [Destroy Fortune]. The Heir of Doom negates the Prince's ability with [Mana Decay]. The Prince gets fed up with the Heir's defenses and pummels them with their weapon until the Heir dies. None of the Heir's other majiks help in any way.
Princes are just better at combat.