I know this was a quest that was never implemented in vanilla. Is there a mod that re-implements this quest in the game?

IIRC Hillsborough estate. http://tesnexus.com/downloads/file.php?id=21353
Or something like it. I haven't tried it.

THat looks more like a farming mod, and doesnt quite grab me. It looks awesome though, just not to my taste. Thanks for the suggestion!

Also, there was a vanilla quest in Cheydinhal that was never finished. Something about a Red Queen or...? Hmm. Here's a link: http://www.uesp.net/wiki/Oblivion:Unmarked_Cave

Anyway, I'd thought I'd piggy back and post it in here with your Duke of Colovia quest search/request.