MetalBard said:
I'm kind of hoping Aristotle explains them.
They've since been touched on, but I don't mind posting a little something as well.
<<disclaimer! I've researched Voodoo on my own time in the past. I don't practice voodoo, nor do I claim to be an authority. My information could be wrong, but should be fine for fictional gaming.>>
Voodoo is the glamorized/fictionalized concept of vodun (sometimes vodou), a religion/culture that combines catholicsm, masonry, and tribal african beliefs. it's primary practitioners are of hatian descent, although most anyone can practice voodoo and there are several very similar groups found in other areas of the south, caribean, and latin america.
A houngan is a voodoo priest, a priestess is called a mambo. The primary method of worship performed by priests is to entice, through gifts (food and liquor) or sacrifice, the Loa (great spirits) into their own bodies. It is ritual, invited, possession known as "mounting".
The Loa (actual spelled Lwa) are powerful spirits, each with a domain they control. The most powerful spirit is Bondye, but preists never worship him directly. They pay tribute to the Loas who in turn pay tribute to Bondye. All Loa have a 'dark' aspect. The dark aspect of a Loa is called a Petro Loa. The Petro loa actually go by different names as well, but that isn't necessary for your purposes.
You can find lists of Loa by googling the word. Many won't serve the purposes you need them to, but a few good candidates exist. I'd suggest Baron Samedi, the petro loa of death and the creation of zombies; or Kalfu, the petro loa of the crossroads who brings about bad luck, deliberate destruction, and commands evil spirits, or Ogoun the "Iron Warrior".
A priest often calls exclusively on one Loa, to avoid jealousy among the Loa, but nothing says yours can't call on more than one. While it may be unfounded in the real culture, and depending on how dark you want to go, perhaps your voodoo priest is in league with a number of petro loa who are looking for a sacrifice worthy of summoning Bondye himself. Either because they want to invoke his wrath, or (because Bondye translates to "good god") so they can destroy him and take his place. Maybe it's best to leave the exact reason a mystery. Either way a carnivale full of innocents might be just the sort of target the priest is looking for.
Or don't go the voodoo route. It was just an idea...