Not to be confused with Dead Gods (who are the definite deceased beings of true gods, like Creators), the Undying Gods are immortals who, having once been mortal, became ascended gods. Using various rituals and energies, a powerful necromancer or other being can become an immortal deity, or an Undying God could refer to a former Living God.
When it comes to the Undying Gods, there are mainly two kinds.
LICH GODS:
Lich Gods are necromancers or already powerful liches who use necrotic energies to become immortal gods. They use the powers of undeath to rule over their worshipers (usually enslaved); living or undead. Lich Gods are mainly found in Ryumar and Aesudarh (Draugr Kings), with some sometimes appearing in the lands of Ashanor and Korash.
On the latter two continents, there were many Lich Gods after the events of the Confluence of Nagal'or, but most of their empires were destroyed by the living. It is uncertain how many Lich Gods still survive in these lands; hiding in secret cults, or sleeping in faraway tombs until they can awaken anew. So-called Forsaken Cults consistent of nercromancers travel the many lands to find these various Lich Gods, and pledge their lives and servitude to them; usually in exchange for power and wealth.
One such Forsaken Cult once saw to the rise of a new Lich God in Avanor's kingdom of Amaranthia. During the Age of Death, Amaranthia was conquered and ruled by a Lich God and his Forsaken cultists, and the lands of Amaranthia suffered to undead and necrotic energies; of which many still linger there to this day. The Amaranthian Lich God was only defeated after the heroic return of the nation's prince, who rallied the people and slew the necromantic masters.
In other parts of the world where Lich Gods reside or have resided, the presence of undead creatures is strong and noticeable. The Lichlords of Ryumar's sands once conquered the many human lands of the vast deserts; including Pyram, Heresh, Dhur Faal, Cairak Amon, Al Saqhur, Nerumar, Khenakka, and others. The Lichlords took these lands for themselves; consuming their people and burying them beneath the sands, where underground ruins became vast and maze-like tombs for sleeping Liches.
Korash's Lich Gods once ruled the vast jungles of Khult in great and powerful empires. Their reign would eventually end when the kingdoms and empires of the living united against them. Today Khult is a cursed land of necrotic energies, undead creatures, and ruined cities. But every now and then a Lich God reappears, bent on reclaiming their old empire. It was the fall of the Khult Lich Gods that saw to the rise of the Grand Sepulcher, which would, among other responsibilities, trap and store the souls of Lich Gods in their necropolises.
In Aesudarh, Lich Gods are known as Draugr Kings; risen to undead life for largely unknown reasons. They reside in the ruined lands of the former Asvard Kingdoms, where the Harrowfrost's cold winds have swallowed the lands and dominate. Sometimes large barrows become cursed or taken over by a newly awakened Draugr King, who uses the many dead of those barrows to expand their kingdoms of undeath towards the living.
Lich Gods are rare in Ashanor, though they used to be many. The clans of Canthar, Shanthar and Tanthar banded together and saw to the end of the Lich Gods who arose after the Confluence of Nagal'or; finally seeing peace and victory after some 300 years of fighting. Today the tombs of the Lich Gods are sealed or empty; hidden away from public knowledge. Sometimes those of the Forsaken Cults learn of the location of these tombs, and try to reawaken the Lich Gods who slumber there.
Lich Gods, like with most Undying Gods, can't truly die. They are effectively immortal, and some could argue that they are more immortal than the Creators themselves. To actually kill a Lich God is considered impossible, as they, like all Liches, use powerful phylacteries to sustain their physical forms. Even if the phylactery is destroyed, the Lich's soul and ethereal body still survives, which is why mortals will instead use certain rituals or magical devices to eternally trap a Lich's soul; containing it and keeping it from ever growing powerful again.
SPECTRAL GODS:
Spectral Gods are the deceased and ethereal forms of the Living Gods in Korash. Their souls won't enter Kelmora's Cycle, and so instead they linger near their ritually preserved remains; often resting and slumbering until a worthy body is prepared for them, and allows them to resurrect as Living Gods.
Unlike when they were Living Gods, Spectral Gods possess very little deistic power in comparison. They will wait and rest in prepared familial tombs that have hosted their various remains for hundreds of years; most of which are kept and maintained by the Grand Sepulcher. As a Spectral God is resurrected into a Living God, the powers that bound them to their old remains cease to exist.
When a Living God dies, his or her remains are brought to one of the Grand Sepulcher's necropolises; the largest being the Spectral Necropolis of Nehekka, in the Scaled Cities. Smaller necropolises can be found throughout the lands of southern Korash, but only lesser remains, like fingers, teeth, nails or toes, are safekept in them.
It is in the Spectral Necropolis that the Grand Sepulcher's deathpriests will safekeep and maintain the icons and remains (usually ashes or heads) of all the past Living Gods, as well as any worldly riches they may have amassed in life. Among the many tombs of the necropolis (old or new, rich or poor), the tombs of the Living Gods are the most elaborate and decorated. They are also the best guarded of the tombs; watched over by dedicated servants or clever traps and other puzzling contraptions.
While the Grand Sepulcher mainly serves Kelmora, they recognize that Spectral Gods can't return to her Cycle in the afterlife, and so they take it upon themselves to watch over the souls of the Spectral Gods, as it is something that Kelmora cannot do. Kelmora dislikes any beings of undeath, including the Spectral Gods, but she more or less tolerates their existence.
The Spectral Gods mainly sleep and wait in or near their tombs; bound to their remains. Those who disturb them before they are ready for resurrection might awaken an angered Spectral God, who will either swallow the souls of the disturbers, or curse them with incurable necrotic afflictions.
Like with Lich Gods, a Spectral God can't truly die. They are without host bodies, but their souls and ethereal forms still linger in the world. Spectral Gods are bound to their remains, like how a Lich God is bound to their phylactery. But if a Spectral God's remains were to be somehow destroyed, their grip in the world would lessen.
At most this still won't actually kill them, but it will weaken them and destroy any mental connection they had to living life. A disconnected Spectral God will lose his or her memories and powers, and can no longer be resurrected as a Living God. Instead they might become vengeful specters; often eternally cursing and haunting those who destroyed their remains, going as far as cursing entire future lineages of an individual's family.