Actually, ID 10 represents neutral factions TOWN_NO_TYPE and the unused ID TOWN_SPECIAL. Using these in their cities breaks the game in the building window.

You can create a hero with ID TOWN_NO_TYPE, but they will disappear from the map when the game starts because they won't receive an army from levels 1, 2, and 3. However, if you organize such heroes using an NCF file, the hero will be almost functional, but you'll need to adjust the dotted outline on the map.