A few days ago I saw a post on facebook from a fellow wondering if anyone had tried mixing Warhammer Fantasy elements into their AD&D 1e game. Immediately I wanted to grab this fellow by the collar, shake him abruptly and yell, "Yes! Yes! One Hundred Percent Yes!"
Now, from the post it was difficult to determine if he meant Warhammer Fantasy Battle, or WFRP, the roleplaying game, but really it doesn't matter. Elements from either system can be used, but obviously it's far easier to translate elements from the fantasy roleplaying system into your existing AD&D or RPG game.
Full disclosure...I've been doing this for many, many years.
Sometime in late 1988 I discovered the 1st edition, hardbound volume of WFRP, and I was immediately hooked on it. The cover art and inside illustration was unlike anything I had seen to date, and while the game system wasn't perfect (what system is?), the game world was original and well fleshed-out and was indeed a Grim World of Perilous Adventure! Character life expectancy was short, the game world was littered with danger at every turn, and the forces of chaos were ubiquitous if not obvious. It was a dangerous place to be an adventurer, and I loved the elements that made it so.
I ran a short campaign with a few friends who were less than enthusiastic about learning a new system, but it was easy enough to sway them. We all enjoyed playing, but soon enough we were all itching to get back to AD&D, a system where the characters survival rate was much higher and everyone felt like they had a better grip on rules etc. AD&D was (and for many folks still is) a comfort zone. I was a bit disappointed to leave The Old World, the game universe of WFRP, and the strange and wonderful images of the forces of chaos stuck with me long after I stopped playing.
Fast forward many (many) years. While prepping for an new group of online (Fantasy Grounds) friends I decide to go homebrew with my game world. We were going to play Castles & Crusades, and I wanted to expose the players, all of whom were older, experienced gamers, to something they were not at all expecting. In order to create a sense of wonder and mystery, of newness in a familiar setting, I began to piece together Thayrun, my own personal Grim World of Perilous Adventure!
My process started (as many of us do) with a map and a few grid locations, and then moved on to the nature of my newly birthed game world. Thayrun was going to be a battleground where chaos had long ago been driven out by the opposing forces of law, but now the seeds of evil left on (and beneath) old battlegrounds were going to germinate and bloom...I placed Skaven (the WFRP ratmen) under a few choice locations. There were sightings of odd-looking creatures at the edge of ancient forests (chaos beastmen), and those who search the night sky have been spreading rumors of green lights on the horizon.
Why the heck not? Sure, there were goblins, orcs, trolls, kobolds, giant spiders, gelatinous cubes, beholders, mind flayers and all of the other monstrosities that normally inhabit an AD&D game world, but now I had the added influence of WFRP and all of the rich content of that game world to pull from. It kept my experienced players guessing when it came to both storyline and encounters. Characters who had never experienced the awful power of warpstone were now going to have to deal with the horrible consequences of contact with the raw chaos stuff.
Here is what I'm getting at.
Do not, under any circumstances, be afraid to add or remove elements from your game world. Pilfering ideas, concepts, monsters, items etc. from other games, game supplements, novels, websites (like mine) and any other source you can find is 100% fine. Sure, you may have to spend a bit of time coming up with stats that make sense, or creating some back-story for why that weird, glowing rock made a character sprout a scorpion tail and eyes in the back of his/her head, but that's no reason not to do it!
Can you decimate your precious game-balance introducing such things into your mostly vanilla Forgotten Realms game setting? Maybe. That's a risk you should be willing to take. You can always adjust back, I mean, you are the DM, right? That's the beauty of homebrew, and why so many DM's eventually decide to create their own game-world. It's going to be a process of trial and error, but the experience is well worth the time.
Below is my stat block for a basic Skaven Warrior. It's nothing special, but it certainly isn't what your players will be expecting to run into when they hit those city sewers...
No. Appearing: 2-20
Armor Class: 8
Move: 10” (100’')
Hit Dice: 2
% in Lair: 50%
Number of Attacks: 2 Claw/1 Bite or 1 by weapon type
Special Attacks: infection (see below)
Special Defenses: none
Magic Resistance: 10%
Alignment: Chaotic Evil
Size: MPsionic Ability: nil
Skaven Warriors are generally the largest and most ferocious of this wicked and twisted race of chaos-mad creatures. Most warrens (lairs) contain from 50-100 warriors. Common Skaven are no more than slaves to the warren leaders, but the warrior caste is comprised of those ratmen who exhibit the physical traits necessary to both protect the warren, as well as take on missions outside the warren as needed. Though equipped with natural weapons at birth, the warriors prefer to wield the curved, jagged short blade known as a Kitchik.
Skaven Blight: These chaos ratmen live in cramped warrens, and not unlike their more common cousin (the sewer rat) they are sometimes carriers for disease. Often in close contact with raw chaos material (warpstone) they can often transmit this disease with a successful claw or bite attack (Save Vs. Poison)
This disease will manifest initial symptoms within 24-48 hours. These include fever, dizziness, extreme fatigue, and vomiting. Foods and liquids will be difficult (if not impossible) to keep down. Fluid loss is rapid, and if the infected is not given treatment at the onset of the disease he will dehydrate and die within 2 weeks.