Foucter: Actually, that custom quest came with the later release the Quest Editor. Since this is a GOG release, and I haven't bought it yet... I don't know if this version is patched up to 1.5b. It'll show the version number on the Options screen. If it isn't, then you can't play custom quests anyway.
At the moment there's not enough information... but here's a link to the last remaining Darkstone site, that I know of, which has my custom quest for this game... along with other quests, patches and stuff!
http://dsdb.it-taucher.at/index.php?option=com_content&view=category&layout=blog&id=39&Itemid=71 Well, I believe it's fully patched, it has 4 difficulty levels. I'm not 100% sure.