And they can be made very easily in either D&D or The Fantasy Trip, with a little judicious house ruling.
This idea came to me when playing around with a megadungeon concept for The Fantasy Trip (by the way, megadungeons don't actually work well in TFT, but that's another issue). I wanted to create a "vault of the warlocks," where powerful evil wizard-kings were entombed, and I thought it would be nice if they were more than just high attribute wizards. The basic idea I came up with is simple enough. In TFT, you have two types of characters: heroes (who have quite a few talents) and wizards (who have all the spells). But what if warlocks are occasionally born into the world -- characters who are (in game terms) both heroes and wizards. In TFT, this means you let them spend thier IQ pool on talents as a hero and then "double dip," also spending the pool of IQ points on spells as a wizard. All other rules apply. This creates a dangerous foe without adding too much paperwork or prep.
You can do the same thing in D&D, and the combinations are even more fun. You simply say that once in a century, a man or woman is born who walks two paths. In 3rd edition, there are already rules available for this: the rules for building gestalt characters. In classic D&D, you do the same. Create the warlock using a "best of" approach when combining two classes. Take the better hit die, the better attack progression, the better spell progression, and so on, and you have your epic villain.
In B/X or BECM/RC D&D, this gives you villains like:
- The Dark Lord: fighter and magic-user
- The Necromancer: magic-user and cleric
- The Grandfather of Assassions: cleric and thief
- The Ninja Master: magic-user and thief
- The Crime Lord: Fighter and thief
- The Anti-Paladin: fighter and cleric
These aren't just multi-class characters. They're powerful foes who can draw on the best of two domains, and they will stand apart. If you want to give them a little lasting power, you can even steal a page from 4th edition and inflate their hit points: instead of giving them the "best of" hit dice, give them the total of the hit dice from both classes. Sure, that's nasty, but let's face it -- your dark lord is eventually going to die at the hands of a PCs regardless.
But he'll also be a foe to remember.