Though I have no been much in the mood for Halloween this year, I have come up with one idea, a class for steampunk mad scientists in AD&D (either edition). It is based on a few existing products, of which some can be found on RPGnow:

The Complete Sha'ir's Handbook http://www.rpgnow.com/product/16873/CGR3-The-Complete-Shairs-Handbook-2e?it=1

Player's Option: Spells and Magic http://www.rpgnow.com/product/16864/Players-Option--Spells--Magic-2e?it=1

Jakandor: Isle of Destiny (hopefully in the future)

Dragon 237 (I have no idea if these are for sale as pdfs anywhere)

The mad scientist is based on the Mystic of Nog kit in the Sha'ir's Handbook. I have mentioned them before- http://d-infinity.net/blog/derek-holland/20-additional-powers-mystic-nog (make sure to read that to see what other powers the scientist may have access to). They are spell casters who use their magic to enhance their own bodies. For the scientist these enhancements are the result of visible implants and grafts. The class takes its spell list from Isle of Destiny and Dragon 237, specifically the spells Ambulate Object (2nd level), Enchant Automaton and Imbue with Proficiency from Isle (3rd level) and Enhance Trait (1st level), Discern Sire (2nd level), Teratism I (3rd level) and Teratism II (5th level) from the merlane kit in Dragon. For the latter set, just replace any reference to animal with construct or machine. And then the DM needs to add in any spell they want from other spell lists. What all that does is allow the scientist to create simple constructs, enhance them with the teratism spells and study those created by others for the secrets of their creation.

Enchant Automaton effectively creates (non-undead) skeletons and the caster must keep one spell slot open, of any level, for each active automaton (sort of like 5e's rules for animated dead). I created a series out of it and the result is one for 4th level, which creates constructs based on bugbear stats, and one for 5th, which uses the owlbear as the base monster for stats. In the case of those spells, the slots needed to keep the animation ongoing are 3rd or higher and 5th. In all three cases, the resulting constructs can be enhanced up to hit die +2 times by the teratism spells.

From Player's Option :Spells and Magic, I looked through the optional abilities and limitations for ways to flavor the class. For abilities, there are Constitution Adjustment (fighter bonus to hit points) and Followers (20-200 0 level characters plus a few low level wizards). As the scientist can enhance their physical ability scores, the chances of meeting one with a lot of hit points for a wizard is probable. As for the followers, they are actually machines- cast offs of the creation process. These weak constructs can not be enhanced with teratism- any attempts causes them to fall apart. So, the scientist is physically powerful and his lab is infested with machines that can gang up on player characters. And, unlike 0 level humans, these constructs may be difficult to discern from the other junk littering the lab.

For hindrances, I picked Hazardous Spells and Supernatural Constraint. The hazard is the implants within the scientist reacting with combat spells (or any spell with a casting time of a round or less) they cast and the result is 1 point of damage per spell level unless they save versus breath weapon. The constraint is as the mad scientist's work is astoundingly unnatural, their combat spells have no effect on clerics and druids. The divine find the scientist's work to be so repugnant that the gods simply ignore it all and allow their champions to do the same. This does not prevent them from using their constructs from beating the crap out of divine casters, it just keeps them from using their spell casting. If you want a different constraint, I was also considering having water as a weakness- the constructs and spells of the mad scientist simply can not function underwater. Note that I didn't bother balancing the character points from the abilities and hindrances- the class is meant for NPCs and they can get away with that.

Now a mad scientist isn't truely terrifying unless they can do horrible things to the player characters or their allies. In this case, the solution was clear to me- reskinned parasite mutations. Those can be found in Wisdom from the Wastelands 12 http://www.rpgnow.com/product/101685/Wisdom-from-the-Wastelands-Issue-12-Parasite-Mutations-Transformations The side effects of the parasitic implants and grafts are the result of imperfect knowledge of how the body (and implant) function. To find a mad scientist who uses implants without side effects should be quite terrifying. They should recieve bonuses in combat versus living opponents, say -2 to saves versus their spells and they get +2 to damage for weapons. And, if they are able to develop implants that allow control (Domination or Charm), then cyborg monsters can be added to their retinue of horrors.

So the result is a character who is personally powerful, has access to magic that the PCs may have never encountered before (esp if the DM creates some spells for him) and leads an army of robots and cyborgs that can crush villages and make siege against small cities. Not to mention all his victims who show the world what has been done to them. I think that works perfectly for a horror adventure.