The Ten Commandments of Villainy--minus Charlton Heston
In the beginning, the darkness came from the valley and spoke unto me, "Dost my will?" and I asked humbly, "I am thine servant Theo, what is thy desire?" The Darkness replied. "Show them, the true power of the dark side, show them what you have been taught of evil through comic books. Lead them into a land of Mountain Dew and Bugles!" And so, I the Prophet received the Ten Commandments of Villainy, to help steer the chosen people from cliches of necromancers and demon lords.
I'm going to discuss making something important to the health and well-being of your campaign. You need to make an adversary. Not just a mere villain, but a dyed-in-the-wool, honest to Pelor son of an otyugh that'll make your PCs cringe with fear, loathing and/or symbolic hatred. So Here is my fairly sensible list of things that are helpful for a good villain.
1st Commandment: Thou shalt not be villainous without motive.
Motive's a big part of the caper. If you want your Big Baddies to be totally awesome, you must think them through at this point first. What do they want? Why do they want it? What are they willing to Sacrifice to get it? Like any PC character with motivations, we're gonna need a backstory, and a class.
Let's say our evil man here is named Thaco (forgive my unoriginality in the name). He's a goblin Sorcerer. Like most Goblins, he was always bullied and treated cruelly by the larger races, and often made as a grunt in small sieges. One day he discover his magical aptitude and the oppressed became the oppressor. He know attempts to unify Goblinoids and other "monsters" into his war machine to retake the world under his banner.
Now we've got his motive and attitude (oppressed with a persecution complex), his plan (take things over), and his method (military conquest, an army led by fear). That was an even two minutes of brainstorming. This attitude will be our foundation.
Note that our "base monster" (a goblin) is very weak here, and did not start in a position of power. He is egotistical, but he's deeply insecure, and is always trying to bully and outdo his opponents though displays in order to prove his superiority. He is not as likely to cut his losses and run as other major villains.
2nd Commandment: Thou shalt always have a plan b.
Make your plans good. Make them deliberate. Make them several weeks in advance. Our evil Sorcerer Thaco got smacked too hard in the first round thanks to a lousy initiative? This is what gaseous form is for. You're arch-villain needs to be able to survive and escape form the most horribly bad thing gone wrong you can imagine, at least a few times, to truly infuriate the players. Leave some treasure and XP behind, but run quickly. I recommend Gaseous Form, Smoke Steps, and teleporting spells, myself. These tools allow a sorcerer to escape his grisly fate without too many problems.
As a sorcerer, he also should probably have a decent charisma and ranks in use magic device, so don't be afraid to throw scrolls, wands, and other random evil artifacts at the players.
Also, consider making them face off against illusions of Thaco at first, before they even really meet him in the flesh.
3rd Commandment: Thou shalt always have minions.
I know the leadership rules are how PCs get minions, but this is one area where I let a Big Baddie be a little better than the PCs. Big Baddies have minions, of differing uses and skills--and I'm talking about more than combat. A good evil mastermind will have more than just minions with high CRs, he'll have minions who can subvert, build weapons, arm the masses, finance operations, and even serve him tea. I love a high-level NPC hired by the Big Baddie to kill the party as much as the next guy, but sometimes it's more handy to have a humble bar-wench and street ear than a CR 10 assassin. Think of the James Bond movies. The villains had all sort of henchmen, from elite killers to low-level thugs, and of course double agents. Think out your minions. A good entourage of minions will help define how your villain works things out.
We've already stated that our boy Thaco here is a goblin who used to work for hobgoblins, and has ascended the rungs of the military ladder to the top of goblinoid society. In this case, it would be wise to stay mostly in theme, having a "secret police" of goblin rogues, and hobgoblins and bugbears for muscle. He may have also recruited other "outcasts" and monsters, ranging from Troglodytes to Orcs, depending on your taste. The key component is the desire of these people to serve their new master unerringly through fear. Like most loyal thralls to a ruthless dictator (I don't care about alignment, probably an evil one, most likely lawful, seeing as it's a militaristic dictatorship), they desire to serve--so they don't end up being fed to his pet Purple Worm.
4th Commandment: Thou Shalt haveth a whole subscription, not just deep issues.
All major Villians must have personalities that are unique and memorable. I'm not very good at quirks, so I talked a good friend of mine into coming up with two without telling him what I was doing. His two were a Fear of Amish people, and Cannibalism (it should be mentioned that you may need to find some really weird people to talk to at random for that method of finding ideas to work I recommend getting some friends at a local community college or mental asylum, but many gaming groups already have this level of nuttiness in the group to siphon off their randomness).
Amish people, form what little I know about them are very spiritual and believe in living humbly without technological dependence, so I would translate this into a fear and hatred of primitive mystics (druids, shamans, dragon shamans, etc.), which he will attack or order the destruction of on sight. We will add this to his back story, by claiming that he was interested in arcane research and alchemy, but the shamans of the tribe ridiculed him and never supported his inventions, which could have protected them form the hobgoblin war-chiefs.
As for the cannibalism, do to his superiority complex, he will only eat inferior, lesser creatures in his mind. A regular punishment for a subordinate's failure is the consumption of a lieutenant's vital organ, let's just say the heart and brain are the parts he relishes the most.
Okay, from those two personality quirks, we can add some depth here. I think there are few things more gruesome than watching a scrying spell on an enemy cut to a meal scene, where it takes them a minute to realize their foiled adversary is now dinner for someone higher on both the command and food chain. This kind of thing will make your players severely worried as to exactly what kind of monster their facing, and if the visions are vague enough, they'll just know it eats its own minions without remorse. This will give them some severely misleading conceptions about their opponent.
The hatred of primitive divine spell-casters will allow a wide range of targets for hatred, and is even better if you have a PC whom Thaco can loathe with every fiber of his being.
5th Commandment: Thou shalt make it personal.
Villains aren't despised and hated for starting wars, or fighting the heroes to the death. They're hated when they cross the line, when they make it personal. Green Goblin wouldn't be half the monster he is/was if it weren't for Gwen Stacey. Chameleon was only a moderate villain, but he was taken seriously when he kidnapped Mary Jane. Lex Luthor's kidnapping of his own daughter was far greater villainy than anything he did to take over the world.
Yes, Thaco's an adversary, but to really make him evil, you need to find someone in the party and give them a true reason to hate Thaco. Killing a pet or animal companion, razing the PC's home village, or even just trying to destroy everything that character holds dear (destroying forests is a really good way to anger a party's druid or ranger. Dragon shamans hate it when draconic races of similar attitudes are being fought and enslaved, etc.). Attacking the things that your PCs love and cherish will cause them a lot of RP motivation to destroy this guy on sight.
6th Commandment: Thou shalt Monologue. And dialogue. Actually, thou shalt run thy big fat mouth off.
This is important. Every villain's gotta say something good to be truly bad. You have to taunt, goad, and tick off your characters to all ends. Exploit their previous battles, their rage, and even mock their very attitudes:
"You didn't defeat me in The Ruins of Cephalon, you will not vanquish me here!"
"Nice outfit, steal off an ogre? You smell like the Backside o' one."
"You call that a spell? This is a spell, DRUID!!!" (usually said while lobbing a fireball)
The taunting and monologue is equally important during combat. That's why being physically inaccessible while in an epic battle is so useful--it give you time for witty banter. Be sure to have lots of minions to distract them while you trash-talk.
Thaco, like most goblins, has a sick sense of humor, and will often use horrible lines to make opponents cringe or get mad.
"Nothing penetrates the Superior THACO!" (Old-schoolers will snort at this)
"You fight worse than a drunken kobold, and you smell like an ogre's hind quarters."
"You remind of young cliche named Drazz't..."
"Soil yourself yet, low-breed?"
7th Commandment. Thou shalt have many other "boss monsters" before thee.
A good memorable villain will have many schemes, and will almost always have some powerful, deadly monstrosities in his service to help things along--at least one per scheme/adventure, if not more.
Again, relevancy's important. Now we've already described goblin(oid) slaves as the major source of Lord Thaco's dominance. Thankfully, most of these big bad bosses can be easily made through templates and existing monster stats, or character levels, and occasional prestige classes.
Since he has no remorse for abusing lesser races, using templates through horrible magic experimentation to create prototypes for conquest is likely. Some recommendations include:
Half-golem (flesh or clay) Hobgoblin
Tiefling Goblin Rogue (given a blood transfer through mystic rites)
Using The Tauric template to graft a Worg's body to a Hobgoblin's body, creating a fearsome leader
A lycanthropic Kobold (were-viper) who has taken levels in rogue.
A Gnoll with ranger levels, serving as a pet to Thaco
A gelatinous Ogre
A bladerager Troll (this is an actual monster, created by foul experimentation on trolls He might have on or two of these, but not many)
Skeletons and zombies of monstrous humanoids, including Minotaurs and trolls, with the occasional gray render zombie or pet guarding a raiding party.
An Athach (because they look more like a magical creature gone awry than anything else.)
The main goal of Boss monsters is to deliver a semi-climactic fight at every adventure relevant to the campaign, at ever CR, so that you've always got options.
8th Commandment: Thou shalt always be as vague and murky as a river of mud.
I'm not saying you shouldn't drop hints, but it may take several encounters with Thaco's troops before the party realizes what's going on--that a goblin despot with immense spell-casting skills was the major villain. Play with the opponent's perceptions of your villain. Make interrogation barely useful. Make the orders covert, and the ideas less substantial. Your villain probably shouldn't unveil himself until he feels somewhat threatened by the party's activities.
9th Commandment: Thou shalt have schemes.
We know the goblin forces of Thaco are organizing for military, so how are they doing it? Raiding caravans for resources? Attacking an infamous castle guarded by the undead in order to acquire a place to rule from? Kidnapping derro and duergar to properly outfit the military war machine? Seeking ancient artifacts of power to increase his own abilities? You must decide what the villain is doing. After all, it's not like these guys sit around and watch soap operas in their lairs until the good guys show up.
Thaco already has magical power, but he needs more troops. So, he goes on "recruiting drives" where he attempt to gain leadership over primitive tribes by raiding local villages to show his prowess as a warrior. After the PCs disrupted one of his raids, he lost some followers, and now is desperate to attack them with some killer shock troops in order to show his skill.
10th Commandent: Death is but a temporary setback.
A good villain can come back from the dead, or merely fake his death. Most people already know about Resurrection form the dead, but fake deaths are less common by villains in most campaigns. Screw coming back as an undead, I want my minion to die in my place. Nothing an illusion spell can't do--or a change of clothes into "leaders' clothing" if they don't know what you look like.
Again, remember that we're being evil here Giving them the false satisfaction of killing the big villain when it turns out they got played, if executed well, has some serous pay-off in RP terms--the party will now truly consider the villain worth more than a second glance.
Thaco, like most mages has many options. He could sell hi soul to a demon lord for resurrection, rely on an evil cleric, or even come back as an undead such as a liche or vampire (a vampire lieutenant is needed for this particular trick to work, claiming he was mortally wounded, and had to make a sacrifice play).
Also as a mage, it's easy to fake a death, either form a simple illusion magic, to a teleport or gaseous form spell before the blast of a spell powerful enough to not leave a corpse behind, to using a minion as a body double. Also, falling off of a steep precipice and vanishing by a magic spell after you hit the water is always a good back-up plan. If there's no water, then simply use a nice illusion spell to make it look like hit the ground when you didn't. Equally helpful.
These Ten Commandments will strengthen your villains, if used well. I hope you use them well. The Darkness knows I will. And I'll cackle when I do so.