Probably post-ancient keeper. The 'Dungeons and Keepers' campaign has some pretty hard maps too, it's not included in the game. Also, if you go for the alpha-builds, there are several bundled maps, many of them are pretty hard.
And making a campaign is not hard no, make the subfolders in the campgns folder, and copy+edit a campaign config file to point to those folders. There's a wiki page here: https://github.com/dkfans/keeperfx/w...a-new-campaign