I've always wanted to have a recurring villain for the party. Being loathe to force a circumstance where the bad guy always gets away, I think I've found a way to do it that provides me some options if the party can actually take out the bad guy before they reach some epic level of power.
The background involves the emphases system for spell research that I can't seem to stop talking about. In this case, the bad guy has an emphasis based on a number, specifically the number four. The Quaternary Acolyte receives a bonus to research (or possibly cast) any spell with that somehow involves the number four. One obvious spells would include a Fireball variation with four smaller blasts. However, there are more options available such as Hold Four Monsters as opposed to Mass Hold Monsters. Squares and rectangles have four sides and four corners; since this is an NPC, I'm taking any connection to number four.
So let's say the party is from the same town. Cattle and livestock are being found mutilated. Outside of town, odd-shaped burn marks appear in fields: one is a pentagon, another is a triangle, etc. Rumors spread of an evil cult. Odd things begin to happen in town that aren't a major threat, but can be a bit unnerving. A few poorly counterfeited coins begin to circulate. Trees on the edge of town sway to a wind that no one feels. A very localized rain storm suddenly appears over a merchant's storefront and soaks everything he is selling.
The merchant believes that an asectic that lives outside of town is responsible. What passes for law and order has gathered a group of locals together to confront him at his hovel.
Feel free to modify, but for me, the ascetic is not the villain, but a neutral character. He will know about a few events that he has seen. Another unnatural event would happen while the mob is "questioning" the ascetic, thereby clearing him and providing clues to the real culprits.
The villains are a strange group of wizards that are experimenting with what they consider a new form of magic. They focus on numbers and their peculiar numerology in magic research. Our villain, the quaternary acolyte is studying the number four. This includes the literal number four as well as any meanings associated with four. Pythagoras considered the number four to be the number of justice or retribution. This could make it problematic for the party as this spellcaster's emphasis will pretty much always apply to them.
What if the party dispatches this one in the first adventure? Introduce the Senary Acolyte. Hexagons aplenty, right? More than that, Pythagoras considered the number six to be the number of creation. With a specialty in creating things, he or she would be quite a formidable opponent. For what it is worth, six is also a perfect number and a triangular number.
This group has the Primary as the leader. Since he or she focuses on the number one, I imagine all kinds of möbius strip inspired spells, including a bridge based on this idea. Then, of course, is a creature based on beign 1-sided and/or a möbius strip...\
There the germ of an idea. Hope to come up with spells and stats here in the next couple of days.