I have gotten very tired in recent years of typical fantasy, most especially epic fantasy. (That is the main reason I avoided Game of Thrones.) And, yet, I cannot shake the awesome appeal of so many fantasy elements. I've been scratching the itch by mashing up fantasy with other genres. Seven Kingdoms is a mashup of fantasy and supers.
Today, I'll talk about mashing up fantasy with an alien invasion. Yes, this was in part inspired by going to see Cowboys vs. Aliens this weekend. But, it also was a setting concept I toyed with many years ago.
Why Do This Mashup?
Leaving aside the "It's awesome!" response, what are some of the reasons to mash up fantasy and alien invasion? The big one for me was to help explain all of those intensely weird creatures in D&D. You have things like beholders, illithid, and aboleth crawling around, with very little else in the ecosystem that even vaguely resemble them. Where did they come from? Well, what if they came from somewhere else? Suddenly, the fact that they are so very different helps to reinforce the setting conceit, rather than undermine it.
Another reason is that an alien invasion represents an excellent frame story for a ragtag band of heroes to come together and save the day. The aliens will likely have already worked to eliminate the top tier defenders of the planet (e.g., Elminster). It's time for the apprentices and squires to tighten their belts and step up to the plate. A few successes will bring them to the attention of the local rulers, who will then be begging them to save them. The typical zero-to-hero story arc of fantasy games fits easily into a scenario of repelling aliens.
Check for Traps
There are going to be some difficulties along the way. There always are. But, in this case, you won't have that many. Your biggest problem, honestly, is going to be working to squash player knowledge of how technology works. (Making the alien technology radically different from our own modern technology will help this considerably, but you will still have the guy who swears that his mage ought to be able to figure out how to use the ships' computers.) Since any sufficiently advanced technology is indistinguishable from magic, just treat the aliens as a different kind of magic.
How Would This Work?
There are a few different ways to depict the aliens. Each has significant advantages and disadvantages. And, of course, I can't possibly cover all the options.
There are three basic invasion scenarios in the fiction: crushing military might (Independence Day, War of the Worlds), surreptitious corruption of the existing structure (Invasion of the Body Snatchers, They Live), and open offers of assistance that may or may not be honest (Childhood's End, V). Any one of these can work well for this mashup, or any combination of them.
You would begin the campaign with a fairly standard fantasy campaign. If you go with the corruption angle, perhaps the PCs discover that there are aliens among us, and have to fight a running battle against a growing conspiracy. This would work really well with aboleths and doppelgangers as the primary villains.
If you go with one of the other scenarios, then great ships land in the capital city in the middle of the first adventure. Maybe they are classic flying saucers, or more like the massive city ships from ID4 or District 9. Or, perhaps they look like regular sailing ships, a la Spelljammer. Regardless, the PCs are as caught off-guard as everyone else.
In a "friendly" scenario, beings step out of the ships and come in peace. Gifts are given, miracles are performed. Do the PCs trust them? Are they trustworthy? If you really want to mess with your players, have the aliens be extremely beneficial and nigh-worshiped by the populace, but they are illithid. The players will flip out, but there will be no in-character reason to distrust them.
In a military scenario, beings burst out of the ship and start killing everything in sight. Beholders are awesome for this scenario. Have some reason why the PCs can survive and escape. Then give them a way to turn the tables. An all-out war against the invading beholder army should be a memorable campaign.
And, of course, combine the corruption scenario with either of the other two for maximum fun. The PCs go to the powers that be for help, only to discover that those powers have already signed on with the aliens. Now who do you trust?