The Necromancer's Path.

Yes, that's exactly how it is. The script only checks the heroes available to the player within the map. A mod to unlock campaign heroes has already appeared twice in the Necromancer's Path. But the first one was more amusing: the player simply attached a bunch of undead to Markel and blamed me for it.

Added 1 minute ago
IchGViji, by the way, how are you doing? I remember you were trying to play through my maps.