There was a rename during 0.8.9a development. The Fifth Horseman is the updated version of The Hero of Etheria. The Hero of Etheria is obsolete. Please only use https://www.moddb.com/games/the-protectors/addons/the-5th-horseman-campaign-for-089b-wbc3-campaign to avoid any issues.
I'm marking this as sticky for better visibility.