The big, bad villain.
You know, your Black Knight, Puppet Master, Wolf in Sheep’s Clothes or Evil Mastermind.
Not every adventure requires one. But if you’re stuck for ideas and you want to assemble a session that concludes on a memorable note, then you can’t go wrong having the PCs unravel the plans of a villain and close with a boss fight.
The trick is coming up with a villain’s scheme on the fly. If you’re a good-hearted person — like me — you might be hard-pressed to come up with the schemes and methods of a black-hearted villain.
Thankfully, there are resources to help.
Regular Stew readers will know I’ve long relied upon the Kalamar Villain Design Handbook  for such work. It’s dependable, expansive in scope and detailed in its examples — just chock full of stuff regardless of game system. The Pathfinder GameMastery Guide  has several pages devoted to villain archetypes you can build an adventure around. The Ultimate Toolbox is full of many charts that cover this ground, but there is a lot of page-turning involved. Trail of Cthulhu’s  recommendations for Spine and Skeleton provide a spot-on framework for devising a path the PCs can take toward confronting a villain. (I’d also be remiss in saying that stories of true crime work too; check out the police reports in your local newspaper.)
Like I said, all good, and they still have utility.
But my new go-to spot are two facing pages in the fifth edition Dungeon Master’s Guide . (And having the charts on facing pages is a big plus. Hats off to the layout and design team for making that part happen.) These are the Villain’s Scheme table and the Villain’s Methods table on pages 94-95.
Better yet, the material is system neutral. These charts will work with any rpg. (System neutral goodies always make us gnomes happy.)
Both charts are branching charts. So, a d8 role on the scheme chart will require a secondary d4 or d6 roll that expands on the result. For example, say a result of 3 puts you on “Magic.” The villain’s scheme involves magic. But how? Well the next roll answers that. A result of 2 means the villain wants to “Build a construct or magical device.” Ah-ha! Now we’re getting somewhere.
The second chart — Methods — is where things get really interesting. The results here are intended to amplify the scheme chart by giving the GM ideas for how nefarious/cruel/villainous the big bad can be. This d20 roll will inform you on how the villain achieves their misdeeds. Assault, coercion, lying, torture, vice, theft, stalking, and politics are all examples of what’s possible.
The secondary roll gives further examples: There are 10 types of theft/property crime, six types of torture, 10 ways to murder, eight types of magical mayhem, just to name a few.
Now, not every result on the methods chart fits as neatly as a puzzle piece with the results of the schemes chart. But between the two, any GM is four random rolls from coming up with enough villainy to make a single rpg session work.
Once you’ve got a villain’s scheme and ploy, the rest is selecting minions, monsters, victims, a crime scene and maybe a hideout and stats for your villain. Drop those in a dungeon, city or surrounding wilderness and you’re set.
So keep pages 94 and 95 bookmarked. You never know when your PCs need a villain to match their heroics.