One useful way of creating a one shot or a campaign is to start with the villain and work backwards. In some of the examples below, you might wish to combine the villains and have multiple appear in the same story. Some might be more powerful than others, or serve different narrative roles, Some villains might have been protagonists once, but have become antagonists now. Understanding the nature of the villain helps when it comes to plotting the adventure and the challenges that the PCs face. A villain who is interested to acquiring gold through skullduggery is going to prevent very different challenges to a villain who is interested to eating the souls of others, and both will be motivated to achieve different outcomes. Some villains might even find that other villains interests clash with their own, and that their particular evil is at odds with another's plans. Anyway, feel free to mix, match and alter the bad guys/girls below.
We all know clowns are scary, it's been pointed out many times since IT was published in the 1980s (a better question, given how uncannily terrifying they are, is how anyone came up with them as entertainment in the first place), and this villain is a variation on the killer clown trope.
The Idiot is a demon or evil demigod that draws from the medieval idea of the jester or the fool, and the middle ages notions of 'idiocy'. The idiot presents itself as a jester in rags, its face painted white, with a small cursed doll known as 'The Toy' which sits in its breast pocket. The Idiot has no greater motivation in life than to cause chaos and to witness destruction, and it ingratiates itself with vain and foolish kings and emperors, amusing them at court but often advising on disasterous courses of action. It simply exists to play games and will hate any PC that exposes it for what it is (thus ruining the fun).
When I started this article, I didn't consciously set out to follow a 'joker' villain with a dark knight, but here we are. The dark knight in this case is the warrior of duty and valour who has long ago broken with the chivalric codes that hold him in check after being tempted to ignore everything they have learned. A dark knight might not be completely amoral, they might believe they are serving a higher morality, as Anakin Skywalker does as he starts his journey to Sith-dom. Dark knights might see the PCs as an annoying obstacle in the way to them completing their destiny, and if a PC is part of the chivalric tradition that the knight deviated from he might try to corrupt them:
"Once I was like you, I believed in what they told me, but I saw through their lies. Join me and I will show you the truth..."
You know, the standard sort of things that untrustworthy dark knight characters tend to say before they reveal who your real father is.
Some villains are just plain hungry and want to devour souls. If the object of the adventure is to avoid your soul from being eaten by a demon, lich or vampire-like creature, then the tone of the adventure is probably going to be more horror oriented. The soul eating villain, who might have reawoken after a long sleep, or might have been turned by a more powerful soul eater (and might have been a former friend or comrade), is rarely interested in gold and other trinkets. If immortality can be granted to them by consuming the life forces of others, then money has very little meaning. Instead, they might be interested in acquiring items that have a different value. A soul eater might wish to find and destroy the weapon that can kill it, or it might wish to find the one item that connects it to memories of its humanity (being a soul eater might be a sad and lonely experience). Soul eaters will view PCs as being a nice source of food, or an inconvenience to its plans. As a creature that has lived many lifetimes and sees mortal creatures as insignificant, it is less likely to take any beef with the PCs personally.
For the obsessed alchemist, the knowledge that they could do something drowned out the question as to whether they should do it. An alchemist might be looking to acquire some rare element or droplet of beholder blood to undo a terrible disfigurement they accidentally did to themselves, and the PCs being an obstacle to that goal have to be removed. The alchemist might need to trick the PCs into crossing into a dangerous dimension in order to retrieve something they need to heal themselves or bring a loved one back to life. The alchemist's most important trait is their genius, they are brilliant and inventive thinkers and tend to make life altering decisions for themselves and others without consulting them (they know they are correct, so why bother). Because the alchemist believes they understand the material laws of the multiverse, they are dangerously certain of their own 'rightness' and they are ruthless as a result
"Mere fools who could not possibly understand my genius and what is at stake must be swept out of the way."
This is a tricky one. How do you introduce a treacherous NPC to the party without giving the game away too soon? In the Jon Le Carre spy novel Tinker, Tailor, Soldier, Spy, one of the main characters explains why MI6 was so seduced with the false intelligence a treacherous double agent supplies. The hero, George Smiley explains that it was rather like an art forgery, the more one pays for it, the less one wishes to believe it is fake. The lesson here is that the more the PCs pay for, or invest in an NPC, the less they will wish to believe they are a traitor. You need to give the PCs the opportunity to build up a long relationship with the NPC and to like them, and to have other plausible explanations when things go wrong. However, at a certain point in the campaign, allow the first seeds of doubt to be sown and to leave them in two minds for long enough to create tension. Simply killing the NPC might not be a good idea, if the NPC knows something or can do something that is helpful to the PCs completing the mission.
Re-shaper of Worlds
Ok, so at a certain point we have to engage in the Thanos/Bond Villain/Utopian mad man/woman villain. The past 100 years has been chock full of people whose core idea was:
"That way is utopia, but to get there we must pay a terrible and bloody price. When I say 'we' it's more of a 'you' kinda gig"
This villain understands that in order to perfect the world, we must first tear it down. They are adept at pointing out life's shortcomings, the injustices and the hardships of everyday existence and can even challenge the PCs to find a better alternative to their bloodsoaked malthusian plans. Often these villains seem like quite compelling figures and they attract others to their side through promising simple decisive solutions to life's complex problems. Things like wiping out half the life in the universe instead of trying recycling or car sharing first.
Similar to the re-setter of the universe is the religious fanatic, who may or may not have a direct line to a deity, but is pretty certain that they do. The religious leader, cultist or acolyte has become convinced of one of two things, either that their cause is righteous and that service to their god is the highest duty, or that even if their god is an evil entity, they will be taken care of in this life or the next. Nobody likes to think of themselves or their cause as evil, the Nazis didn't think they were the bad guys even when they were carrying out the most horrific acts; eveyone has a way of justifying themselves on some level. The religious fanatic in the service of an evil god might have a doctrine or a story that tells them that the evil god that was cast down into the underworld was actually unfairly treated by the great celestial hypocrite on high and serving the horned one really part of a process of cosmic rebalancing. Invariably with this character, the evils are the product of what they do when they think they are right.
Emperor with a Destiny
The third of these blood thirsty visionaries is the emperor or king with a destiny. Nations and empires rise and fall, and the leaders of those kingdoms in decline invariably dream of the glory days that their forebears enjoyed. Imagine an emperor of a much diminished empire who looks beyond his borders and seeths with resentment as former imperial territories thrive and prosper. How easy would it be to undo the decades of decline, humiliation and unrest at home by taking back what is ours in the first place. Perhaps this hubristic character, who isn't attune to history's verdicts, might be helped by an evil mage consigliere, or he might seek out a weapon that would even the odds and bring independent kings back under the control of the empire.
Gangster on a Mission
Sometimes it is good to take things down a level and to run an adventure on a much smaller scale. If so, they gangster boss building up their empire is a great place to start. If the PCs are in an urban environment, they might interact with a low level rogue who has dreams of glory and power. In order for this to make sense, you might need to create a criminal trade for this individual to try to monopolise. The problem with a thieve's guild is that stealing kerchiefs and fob watches will only make one so wealthy, most modern criminal empires aren't based on theft, but on the control and supply of illegal things (drugs, for example). In a medieval fantasy setting, this sort of gangsterism might be slightly harder to generate, but a possible example might be:
In a port city where the authorities rely on taxing people's work to continue paying for the king's wars, life is pretty miserable. An enterprising rogue heard that in the caverns in the mountains, strange black crystals grow which when touched, allow people to enter a dream like state that takes them away from their pain. Harvesting these crystals soon becomes illegal, but people are desperate to be supplied them and will pay everything they have. Some impoverished people sell their children into slavery to buy the crystals, and the poorly paid city watch become so corrupted that they stop even trying to enforce the law. Soon other gangsters want a part of the trade and they come from other cities to seize the crystal supply and take it back with them. They kidnap scholars and miners to go deeper and deeper into the mountains to find new black crystal caverns. All the while the streets of the port city are littered with bodies as both sides fight it out and the authorities are powerless to stop them .