Underground dwellers.

It doesn't fit into the lore, and there are already many variations on this theme. Besides, the underground enemies of the Elves are gnomes. A Warlock is an Elf hero, and it's silly to call a unit that. In general, there are many such ideas, but they won't fit into the universe; otherwise, the Dungeons would become quite populated. At least they haven't started populating Shi'o with new races besides demons yet.