To achieve this state, an individual must've been brought back from the dead or killed in a manner that allowed them to be "reborn". Magic was often involved. Generally speaking, undead beings had some alteration to or lack of a being's form or faculty.
Post-death, an undead whose form was constructed by various peoples' body parts. The standards of living generally change; for example, vampires and zompires gained the desire and need to feed on mammalian blood. Furthermore, they lost their need to breathe, and their hearts ceased beating. Vampires retained their core personalities and ability to think and make decisions (though the impact of this is lessened greatly by the loss of their souls).
Zombies, on the other hand, lost their conscious mental capabilities and essentially become walking corpses. Ghosts were the souls of once-living people who were stuck on earth. They were incorporeal, but possessed the minds of the people they once were, such as Dennis Pearson and Matthias Pavayne.
- An Incan Mummy Princess who was ritually killed and mummified, returned to life because of a cursed seal and became a living girl. After she stopped feeding on the life forces of others, she reverted to a mummy and died.
- Warren Mears was brought back as a skinless being who could only be kept alive with magic and the end of which led to his second death. This did not apply to vampires, but did prevent them from reproducing, instead forming brainless "zompires" in their place.
- Wolfram & Hart had a standard perpetuity clause in their contracts which allowed their employees to remain in the firm after death as undead, immortal beings with the signs of their death still visible.
- Chris Epps brought back his brother Daryl through a scientific procedure which left him in a Frankenstein's Monster-like state.
- The 314 Project was a military operation aimed at creating a race of Bio-mechanical demonoids, where dead soldiers would be scientifically reanimated with cybernetic enhancements and demonic body parts.