Heroes 5.5 WarGame Edition - Tactical Edition.

The same applies to the fifth game. Agrael, for example. Godric's brother, Aidan, almost succumbed to corruption. Perhaps Jezebeth was an elf before. In fact, I think infernals are good recruiters, and many people switched to Urgash. Or, to be more precise, that's one possible explanation, but it doesn't contradict the lore.

Agrael is not a human, but a High Elf (originally, he was even a Low Elf). Aidan didn't willingly choose this precarious path, and as soon as he realized what was happening, he became a dragon knight. Jezebeth was a human (remember the human campaign in Heroes 6).
- Well, that's according to the lore (this lore is already getting tiresome - sometimes I just want to take at least one step in a different direction).