Today I want to go over how to go about doing such a thing with an example I have thrown out to a few people in the past year or two. For this example we will be using 5th Edition D&D, because it has a good amount of options, but not overwhelmingly so. You have magic users, combat experts, and all kinds of things in between. What doesn't the 5th Edition Player's Handbook have? Witch doctors.
The voodoo priest / witch doctor is an archetype that is not covered very often for players. Pathfinder does have a version of it in the voodoo oracle build. 5th Edition D&D does not have anything that really matches it though. There are, however, a number of classes with features that would belong with a witch doctor. Now you can go ahead and develop your own class from a mix of features, but that requires design and testing. The easier method is by using what is already there in the right way.
To start let's list some of the things that make a voodoo priest stand out. And before that I want to mention that I am going with the more pop culture aspects rather than getting into the technical differences between voodoo, hoodoo, and other related practices.
- nature is a big deal
- aspects of nature are deified
- death is very important
- zombies are a thing
- live in forests, swamps, graveyards
- herbalist medicine
- strange magic
Now that we have a could starting place, let's look at what we can use to fill in these aspects of the witch doctor. We can start by listing some obvious choices. For the nature-related things we could go with druid, nature cleric, or perhaps ranger. For the zombies and death we could go with necromancer or death cleric. All of these include magic, but we should probably remove ranger because, despite the high nature relation, there is very little magic and a bit too much combat focus.
That narrows it down to some form of druid, wizard, or cleric. If you ask me, the best way to create such a character where you need a mix of abilities is to multiclass. Of course you need to be careful with multiclassing. While 5th edition has some very beneficial methodology here, especially with spell slot figuring, it overloading in class number degrades your abilities over time. For those who don't understand why there are two reasons. The first is that the more classes you take the longer it will take to advance to the powerful features of any given one, and less likely you'll achieve any of those. Secondly, because ability improvements or feats are given by class level (not character level) it will require more time to get any one of those compared to a single class character. In the end I suggest 2 classes for multiclass characters and, for those who are clever and willing, 3.
Where does that leave us for the witch doctor? Personally, get cleric our of there. There is a bit too much basic religion involved in clerics. Churches, the classic idea of gods and goddesses, and organized ways of thinking. These don't fit a much more loosely defined, personally acting, witch doctor. Let us take the druid and the wizard. The second of these is the easiest to work with. Simply choose necromancer as your school of magic. The first, I feel, is also fairly easy. Don't go with the shapeshifting build, that doesn't fit properly with what we are building. Instead go with the Circle of the Land. Then choose swamp. Not only does swamp fit the dark atmosphere of witch doctors, it also has bonus spells that fit in nicely. You may say that those bonus spells can be obtained with wizard, well so can many of the ones from other lands. However, the wizard aspect of the build is meant to focus on necromancy, so this helps free up choices when selecting wizard spells.
Finally, we can fit in even more of the feel of a witch doctor by selecting the Hermit background. This will grant you proficiencies in medicine and religion which both work well for the witch doctor. Not only that, it also grants you a proficiency in the artisan tool herbalism kit. As for the background feature the discovery can involve information about the forces of nature, specifically looking at the concept of Loa. I would even use the troll's religions in the Warcraft universe as a great magical fantasy example.
From this we have the nature, herbalistic medicine, strange religion, and necromancy of your classic witch doctor. The key to keeping your character skinned as a witch doctor at this point is you. Role-playing will be key, as will spell preparation choices. Most druid spells work just fine and you can focus on whatever aspect here you went through your daily preparation. As for wizard there are a lot of things that may not seem like a classic choice. Make that part of your personality and character development. Focus on choosing necromancy spells and others that fit the feel of your witch doctor. Perhaps you can even work with your GM to remove some spells from the spell list to add in a couple from other lists. Hex and witch bolt from the warlock list would be fantastic choices and giving up mage armor and some other classic wizard spell may be a fair trade. For already available choices stick with spells like: ray of sickness, tasha's hideous laughter, melf's acid arrow, and fear. As a final note I suggest a fetish (like a voodoo doll or bird skull of some kind) to be used as a focus instead of the standard druid ones. This is all flavor so most GMs won't have issue with it.
As you can see, there is a ton that can be done with that one book. There is flexibility built into the classes too, and no reason not to get creative. Think about multiclassing the eldritch knight and ranger to get an arcane archer. Maybe you can combine an elemental monk with druid or sorcerer to really get the Airbender feel going for you. You could even combine assassin with the avenger paladin for a hitman of the gods. The possibilities are endless!