Knowing the origins of undead is often a basic part of their identity and a good way to distinguish ours from what’s come before. A good story excites the imagination. In this case, we’re talking about an undead who wasn’t caused by the bite of another undead, for example, but an original undead of this type.
As with monsters, undead can be created by any number of accidents, whether natural, supernatural, or technological. This might be the most common cause because few people want undead to exist and have the ability to go around creating them. Undead created accidentally are likely to be few in number unless a large-scale event created many of them at once. How many accidents produce the same results? If we want many spirits that are the same, and in the same area, a large accident is one way to justify that; historically, our depictions of spirits have a tendency to show them as largely solitary. A type of spirit that works in groups could be a novel approach.
We might decide that there are certain types of phenomenon that are known to create specific types of undead. If those phenomena are rare but still somewhat widespread, the resulting undead can be as well. What if someone has harnessed that phenomenon and can unleash it on purpose? That could make this undead type more common; this works for monster creation as well.
Undead By Design
It’s safe to say that anyone who purposely creates undead is up to no good. Our perpetrator might be able to control the result. If so, we can decide who wants to do this and why, then figure out the resulting undead attributes, or do this in reverse. Once someone has created one, they don’t necessarily need to continue doing so for them to propagate. Instead, the undead may have the ability to create more of themselves, making them widespread despite having a single, original source.
If our perpetrator cannot control the resulting undead, he might be unhappy with the result. Did he try to destroy it? Chase it away? Did it retaliate and kill or wound him? Or best of all, turn him into one, too, possibly under its control?
Does the creator have control over his undead? Do they obey? Chafe at this? Or do they seemingly like it? Are they crafty enough to pretend to obey only to look for an opportunity to attack him? How does he control them? A device? A spell that’s still in effect and can be nullified by a zone where magic doesn’t work anymore? Or did the means used to create them make his control a given?