Here is the god upon which this campaign is built:
Nethrancor
Bone Giant, Death Father, Keeper of the Core, Spirit Caller
Intermediate Deity
Symbol: A dragon skull with flaming eyes
Home Plane: Hades* (The Grotto of Death)
Alignment: Lawful Evil
Portfolio: Darkness, death, secrets, undead dragons, undeath, underworld
Worshipers: Draconic undead of various types, some humanoid tribes, isolated giant clans, advanced lizard men
Cleric Alignments: LE, NE, LN (adepts and clerics)
Domains: Darkness, Death, Mind, Shadow, Undeath, War
Favored Weapon: Bite (undead dragons), scythe (humanoids and giants)
*Or any Evil or Neutral Evil plane.
When in his lair, Nethrancor (neth-ran-kor) most often appears as a Colossal dragon, with black scales so ancient they appear to be almost gray. His wings are leathery and make him seem even darker when they are unfurled and outstretched. Outside his lair, he usually appears as a huge pair of serpentine flaming eyes. To his worshipers of humanoid or giant type, sometimes he assumes the form of a Huge robed skeleton of any type of giant carrying a scythe. It is unknown what form he uses for his undead dragon followers. In all forms, his eyes flame and smolder. As the Death Father he knows the specifics of all means of unlife and he is more than willing to trade such secrets to gain another sycophant follower. Nethrancor always uses his salient divine ability of Lay Quest on such potential followers to ensure they will worship him properly by becoming undead. He also makes deals to hand out some secret of undeath in return for a service, but only to undead.
Canon
Nethrancor is the draconic deity of endings, afterlife, undeath, and secrets. He provides the secrets of how to become immortal through undeath for mere subservience and alliegeance to him. Many dragons near the end of their life have pledged fealty to him in order to learn some of the secrets of undeath. Most giant undead have made the same bargain, and it is not unheard of for a human or humanoid lich to worship the Death Father. Those who have already moved into undeath can hope to offer a service for some secret of unlife.
There is only one holiday associated with Nethrancor, but it has been known by many names. The Feast of Samhain, Hallows Eve, The Day of the Dead, All Saints' Eve, The Blood Moon, Dead Dawn, and The Hollow Night, are a few of the names it has been called over the centuries. Bardic songs tell that it is a day where daylight does no harm to and has no effect on undead. Undead usually exiled to only rule the night, have one day to dance in the sunshine. It is also a popular time for dark rituals concerning undead animation or lower planar creature summoning.
In game terms, vampires and other undead usually exiled to the night are able to move about freely during this one day. Undead animated on this day gain extra 1 hp per Hit Die, and lower planar creatures summoned lose their ability to break free of the spell through their magic resistance. A strange quirk of the ritual requires that none of the blood of any of the sacrifices be drank.]
Kalisstan, the celebrated sage of the south, imparts a hint that the holiday is mostly celebrated by vampires as their one time a year to watch the sunrise. Those celebrating this holiday usually dance in the streets, watch sunrise and sunset, and gleefully cause as much havoc as possible in Nethrancor's name.
Some places developed a habit of dressing up as ghouls and zombies to confuse and confound the evil denizens that walk the earth that day, making it more of a holiday than a horror day. Other times saw the gifting of candies, cookies, or once even garlic buds to any stranger on this day. It was thought that the roaming undead could not kill someone they were beholden to until they repaid with a gift of equal value. These practices always quickly died out because they did not offer any actual protection from the undead.
Today, in most places where the holiday is "celebrated", the day is spent barricaded at home or in the local temple, going outside only when absolutely necessary. Most families spend The Hollow Night or whatever the holiday is called in that land at that time, telling scary stories around the fireplace, sometimes in rooms fortified with garlic or silver mirrors, always with ale or some other spirit.
The holiday has no set date; instead it is caused by a year-long ritual held once per month, on the night of the new moon. On the thirteenth night of the ritual (1 year later), the holiday starts at dawn and encompasses an radius of five miles from the sight of the sacrifices. Each session of the ritual takes several hours and requires a sacrifice of some kind. All sacrifices must take place on the same sanctified sacrificial altar. The year's sacrifices are divided by the seasons into 4 triads. The first triad of sacrifices must be: wild animals. The second triad of sacrifices must be: befriended animals (pets). The third triad of sacrifices must each be: intelligent creatures. And the last triad of sacrifices must each be: an intelligent creature well-known by the creature performing the ritual. The thirteenth night requires no sacrifice unless the caster wishes to continue the holiday next year (then the sacrifices must start over with at least a wild animal). It is always permissible to use a later sacrificial requirement for an earlier one (e.g. use pets for the first six sacrifices).
Only areas with a strong alliance of death priests, cult of vampires, or cabal of necromancers have this holiday for more than several years at a time (usually hoping to command, control, or forge an alliance with a light-impaired undead clan).
When the holiday first starts though; the general populace will be unprepared for its effects and many will be lost the first few years until sages or bards bring enlightenment, the ritual sacrifices are halted, or the land is overrun with undead.
As Keeper of the Core, Nethrancor trades one secret of undeath for a service from an undead. The only deal he will make with living creatures is to trade the knowledge of one method of becoming undead in return for using that method to become that type of undead in his service.
Clerics and Holy Sites
The Spirit Caller considers the lair of any undead being that worships him to be a holy site. No shrine or temple is needed; indeed, he only has two actual temples: The Grotto of Death (also known as just “The Grotto”), his demesnes in Hades [or any Neutral Evil plane or even the plane of the afterlife], and the frost giant built Krag'r Kompf hidden somewhere among the peaks of the Misty Mountains [or any other snow-capped mountain range].
Giant and humanoid clerics to Nethrancor wear linen arm wrappings called phylacteries, most sport Nethrancor’s symbol on the left breast of their black robe and go about with the hood pulled up to shadow their often dessicated undead face. Undead draconic worshipers of the Keeper of the Core usually have carved statuettes of Nethrancor’s unholy symbol prominately placed in their lair; but other than their undead state, they do not otherwise advertise their choice of lord.