Death Exclusion Zones are areas where Death, herself, as well as her vanguard who have died once and no longer have a fully mortal body cannot enter or get trapped. The souls of those who die inside them do not move on to reincarnate and end up coming back as ghosts or even wraiths. The source of a DEZ could be a power spirit of someone who passed away, or it could be from an old artifact that malfunctioned and formed the proper runes to create such a zone. Yet, there are plenty where individuals have done rituals with runes to create a DEZ.
Unsavory individuals will use runes to bind souls to bodies to raise skeletons and zombies with some semblance of will, but they cannot leave the zone of their own free will. For GMs who want to use undead with Zombies, and animated skeletons, treat them as spirits using the body as a host, armor, or a meat suit (meat mech?).
Any undead or living thing that dies in a DEZ will come back as some form of undead, sometimes stronger than before. If it had a body, it will animate it like a zombie or skeleton depending upon the condition of the body. Once the DEZ is removed, many weaker undead will pass on within a week, however, stronger ones will need to be defeated, or even trapped for Death to deal with.
How does one remove a DEZ?
Primarily, the main source needs to be destroyed. In the case of powerful spirits, they might not pass on when defeated and need to be trapped for Death to help the spirit pass on. For those created artificially, the runes, typically made of blood, silver, and diamond dust, need to be wiped out of existence and the ones etched into surfaces be marred, melted, or otherwise altered to destroy the destroy them. In the case of those made into a crystal, they need shattered.