Back story, in brief is that Gibbering Horrors from Beyond Time™ are making it into our world. The horrors are spiritual and they need bodies to infest. They use the dead.

As each stage progresses, older zombies can arise and, like the DV, the older the zombie, the more powerful it can be. So, the first stage, with the recent dead, will be the shambling dead. As time goes on, older and consequently faster or stronger dead will arise. Also, those bodies that have certain special circumstances might have different capabilities, ie. mummified bodies will essentially be nearly impervious to harm, smarter than the average zombie, and especially susceptible to fire.

There are also necromancers who can control the dead, but they are very rare and very far between.