“Heroes of Might and Magic V: Tribes of the East”

and besides, has it ever happened that a demon was offered light magic?

Yes, and it's one of the strongest builds, specifically focusing on light (Massive Flash and Righteous Strike increase the army's strength by approximately 70-90% of its initial value. And of course, teleportation. It's especially good for Grok, although he's generally the strongest Inferno hero, so 8)).