When designing a campaign with an overarching storyline the most important thing you as a DM can do, is create a believable villain. It is easy to get caught up in the world building, maps, and all the cool ideas. The Big Bad Evil Guy/Gal (BBEG) is the piece that ties it all together. Without a well developed BBEG and their minions, all of the world building and mapping are just soulless backdrops for rolling dice. A BBEG brings flavor and stakes to that village they are visiting or some quest to find some object or piece of information before the BBEG does.
The villain is the hero of their own story
Wait. What? I thought the PCs are the heroes. They are of their own stories.
A good BBEG thinks what they are doing is right and making the world better. Their goal might be the same as the PCs, like unifying a nation, but their methods differ. The PCs might be looking for the rightful heir to the throne or building consensus while the BBEG might be a conquer. The important thing to remember is the BBEG doesn’t think he is the bad guy. Maybe she is the Spare and tries to prove to her father that she would be a more adept ruler than the Heir, who really doesn’t want to rule anyway. She ends up trying so hard to prove that she can do it, but messes it up and keeps going with the plan that doesn’t work.
Gul Duka, Prefect of Bajor from Deep Space 9, wanted to help the people of Bajor come to accept the occupation and being part of the Cardassian Empire. He abolished child labor, increase food rations and cut back on quotas in the camps. His actions saved lives and made the occupation more bearable for the people of Bajor. However, the resistance couldn’t see what he was trying to do and sabotaged his good works, so he had to ruthless suppress the people to get them to see the light.
Be internally consistent
How many times have you watched a movie or played a game where there was a really cool bad guy that seemed like a real serious threat to the protagonists only to do something so completely moronic and out of character you wanted to punch a puppy?
If your BBEG is a methodical plotter whose emotions are not easily roused, then in the penultimate battle goes into a blind rage out of nowhere, then they are not being internally consistent. Or if you create someone who spends years wanting to tear down the world to watch it burn only to become a leader of a highly organized military state for no reason. These types of characters ruin the immersion of the story. A good DM won’t let their players do things out of character, so why would you do it?
Evil doesn’t mean monstrous or destructive of everything
Not all of your BBEGs have to look like monsters or want to destroy everything good or pretty things just because they can. Those types of characters get boring and are more suited for lieutenants of the BBEG. This particular one is a little more subjective than that the others. For BBEGs that are looking to take over the world and rule it themselves, then no reason to go over board with the “evil acts” for shock value. Sure the occasional “object lesson” in brutality can be effective but if there is nothing left of the world when they are done, what’s the point?
Some of the most insidious evil is the cultured evil. Lex Luthor, Hannibal Lector, and Admiral Thrawn are some good examples.
Give them some redeeming qualities
This one ties back to the previous topic. It’s ok for your BBEG to be funny or charming or have one of those personalities that people just like to be around. A dour bad guy that scowls all the time is really one dimensional and gets old quickly. Giving a little quirk to your villain gives them depth, humanizes them and makes the world they inhabit much richer.
Think of Heath Ledger’s Joker character. Even though he was batshit crazy, he had really magnetic personality with a simmering hint of menace. The Joker was blunt, there was no sugar coating what his goals were, but you were still drawn to him to see what he would do next. Negan, from The Walking Dead, is charismatic and he leads a cult of personality. He’s brutally honest, does what he says and has a smile even for his defeated enemies.
They got to their position for a reason
Most of the times, your PC’s will go up against a BBEG, who already has established them in some position of power. Which means they are competent at something. Remember that and don’t muck it up.
Sometimes they are looking to expand that power or consolidate what they have. Either way, they got there somehow. Your BBEG is a warlord who conquered nation after nation and leads through force. They aren’t going to fall to some 1st level characters who stumble into their throne room. If the BBEG got to her position through cunning and guile isn’t going to get surprised by a group of bumbling PCs who manage to thwart scheme after scheme.
When you create your BBEG how they got to where they are is a blue print for going forward. Don’t be hemmed in by the past though. If they BBEG took over a country by force 20 years ago, then its ok there to be a large spy network in the kingdom or to have some advanced weapons/tactics out of the army.
Your BBEG is the thing that ties all of your storyline together. Put as much or more time into creating the BBEG and their power structure as you would anything else in your world. The more details you have the more flavor you can to your world. A well crafted villain will be something your players remember for years.