I am looking for maps for Heroes III.

Yes, there is such a map. "Heroes III - The Restoration of Erathia", map "Unholy quest". You are given a castle on an island, you cannot build anything, and you have one hero, with whom you go down into the dungeon. Then, either you defeat the main villain in the same dungeon, or you go to the surface, rebuild the castle, and then go back down.