Undead

If you are insane enough to travel the old cities, make sure to bring fire with you. Simply hacking and slashing the hordes of undead will not keep them down. - Abeni Meadowwind

Overview

Undead- also called "zombies" -are often found in ancient ruins as those areas are often mass graves. They are the reanimated remains of those killed during the Great Collapse; and yes, that means that the undead do not need to be in one piece to walk the world. Fire is recommended if you wish to kill the corpses permanently though it is by no means the only way to do so. Some are slow walkers while others are quick runners. Dungeon delvers will need to pay particular attention to the latter. In terms of intelligence, most do not display higher cognitive reasoning present in sentient species.
The Undead are motivated to consume the flesh of the living as well as to increase their numbers by infecting the same. They are not picky eaters as they will pursue birds, fish, and beasts as well as sentient humanoids. When there are no living beings within their sight, they hibernate. Most undead will hunt the living as part of packs, with only a few kinds preferring to hunt solo.
There is a widespread belief that the undead are most active at night. This is false, there is no discernible difference in activity between night and day. If an undead is hungry they will hunt.
There are many varieties of undead, some of which are listed below:

Fresh Corpses

These are the undead of unfortunate travelers and adventurers of our present time. They are often found near the outskirts of ancient ruins.

Ancient Corpses

This kind of undead has walked the world since the beginning of the great collapse. The flesh that remains on them has usually been preserved in some fashion to survive the ravages of time. Petrification is a common means by which this happens as is mummification. Some have been desiccated while other bodies have been frozen through various means and have only recently been thawed from their icy storages.

Dismembered limbs

Hacking and slashing a body does not kill them outright. Hacked off limbs will reanimate themselves after a short period of time.

Skeletons

Sometimes the flesh of the undead do not survive the ravages of time, leaving behind only a reanimated skeleton. This seems to be the strangest of undead as a skeleton by itself should not be able to stand much less move on its own.

Note(s) from the Archivists:

Remember the skeleton we found in the library basement? That was fun. - Archivist Theta
"Fun" is not how I would describe that day. - Archivist Enna