|One of Fiend Folio's best monsters|
The main value in establishing these relationships is that it gives the DM a strategy on using them in an encounter; they have built in motivations and goals. Hellish undead tend to guard locations or perform assigned tasks, especially where it can corrupt or subvert a location of significance to the cause of good. Demonic undead are rapacious and destructive; they seek to spread carnage, or create more of their kind by spreading the curse of undeath.
This week I'm turning my attention to the undead creatures of the Fiend Folio and Monster Manual 2. The split between hellish and demonic undead is subtle enough that the writers of the succeeding monster books didn't always pick it up, so part of this exercise is seeing how they can be made to fit.
The Penanggalan is that female vampire variant of a detached head with trailing entrails, pictured above. Vampires are chaotic evil and associated with the Demon Prince Orcus; on first glance it doesn't make sense for the penanggalan to be Hellish. But the writer does mention the penanggalan returning to Hell after death, so the alignment and association was intentional. The folklore of the penanggalan, Malaysian in origin, mentions the penanggalan as a practitioner of Black Magic arts that is cursed after breaking a deal with the Devil. The penanggalan appears as a normal woman during the day, so I would have the creature use subterfuge and trickery to carry out its hellish mission; this at least reinforces the theme I place on devils regarding silver tongued lies, trickery, deceit, and corruption.
The poltergeist fits in with the other unsubstantial undead; it haunts a specific area, the place where it was killed when alive. I can get behind the evil soul, trapped where it died, rattling chains and harassing the living. Early references in the folklore even refer to them as "invisible stone-throwing devils".
The apparition is an ethereal terror that causes massive heart attacks with a psychic choking attack. Seeing as it got the Chaotic Evil treatment, I'm going to say they roam the astral plane indiscriminately, attempting to kill anyone they meet instead of haunting a single area like the Hellish spirits. Chaos is freedom! An apparition is essentially a destructive demon in undead form.
The coffer corpse is the fake-out undead; it appears to be a dead body on a funeral barge, then it pops up and attacks! ZZzzzzzz - kinda boring, I don't ever recall using one of these. Maybe when funeral rites are incomplete, the disinterred body can temporarily host a demonic spirit; thus the coffer corpse wreaks havoc with murderous glee when disturbed, and reinforces the importance of last rites, blessings, and burial rituals.
Death Knights would seem to be loyal servants or tyrannical rulers that understand the power of Law, but the text explicitly says they were created by Demogorgon, Prince of Demons. I'd use them as powerful and destructive lieutenants of nihilism.
I love the Huecuva, and they make perfect sense to me as demonic undead; their touch spreads disease, and the Huecuva of South American folklore is also associated with disease and misfortune.
Revenants and Haunts are both spirits that linger in the mortal world because of unfinished business; they have no ties to Hell or the Abyss. The other new neutral undead were the Animal Skeletons and Monster Zombies, both created by Animate Dead. The Juju Zombie isn't created by Animate Dead, but is made whenever a magic user kills someone with the Energy Drain spell; it's still a servitor.
There are a few undead that are specifically tied to an unnamed evil god or demigod instead of demons or devils; they are the Skeleton Warriors and the Sons of Kyuss. The Skeleton Warriors are powerful cursed servitors, neutral in alignment. The Sons of Kyuss are pretty interesting though; they spread disease, and they have those hideous worms that infect opponents and burrow into the victim's brain. That's just nasty stuff. They're Chaotic Evil and would work fine as demonic undead. However, anyone around during the 3E era probably remembers Age of Worms in Dungeon Magazine, and the adventure path dealing with the return of the elder evil, Kyuss. In an old school game, I'd gladly promote Kyuss to Demon Prince trapped on the material plane, and use the 3E ideas whole cloth.