I think I know what you're looking for: Have you considered the Spellforce series (1 and 2 plus expansions).
Originally I thought it was another run of the mill D&D clone, but I actually found it to be an exteremely good mixture of Settlers, phantasy RTS, and Diablo, which each of those components balanced beautifully.
Had me glued to the screen for a very, very long time. Highly recommended. And please try not to be put off by any negative reviews. I don't think those reviewers took the time to consider or understand what gem they were dealing with.
The multitude of maps plus the map generators give it incredible replay value.