I believe there was a similar post made previously about this issue, but I cant seems to find it so I'll post it again, since update 1.5.4 (I believe) it has been effectively impossible to make custom main maps. When you try to paint on the main map (or any file named main map) it will crash, additionally applying material maps will have the same effect. When generating Navmesh grids the new navmesh will not match the terrain of your map, instead it will continue to match the terrain of Calradia, even if you have deleted the terrain and created new terrain from scratch. When playing your main map ingame, your player model (and all other models) will not travel on the land, instead they will be below or above the terrain. Finally, when you are placing entities on the main map, they will snap to a location below or above your terrain. I am not a coder, but several of the coders who I am working with on several total conversion mods have agreed that there seems to be a strange issue where the game has seemingly hardcoded terrain information associated with the main map, and that any attempt to rectify this incorrect terrain information crashes the game. I believe what I have posted here summarized the current issues with custom main maps, that unfortunately make it impossible to have a true total conversion mod. These issues have persisted since 1.5.4, and are still present on 1.5.7.