Ok not sure if this is an issue with the mod or the game itself. I downloaded the Island of Despair mod from the mod borwser. It's which is supposed to be a new 9 level episode. When I launch Hereteic or hit play from the mod menu, I don't see it at all. Just the regular episodes.

It's probably not supported (yet). The problem with the mod uploading is that you can just upload stuff without it being verified to work first. The only mods that work 100% are listed in the "featured mods" menu. Everything else is a 50/50 chance.

I didn't even see anything in the featured mods menu.

It's visible when you run the game in GOG Galaxy. Must be a bug that it's empty when you run the game the normal way.

You can see the two working ones (Shadows of Chronos, The Iron Forge) in the mod browser though.