The deity of chaos and destruction.

 
The most ancient foe of the universe, Tharizdun is the Father of the Abyss and the Lord of Madness. Tharizdun is the god of Eternal Darkness, Decay, Entropy, Malign Knowledge, Insanity, and Cold. Tharizdun is a long-forgotten god who craves nothing less than the unmaking of the universe, destroying everything—himself included—in the process. During the Dawn War, he created the Abyss, an act for which he was imprisoned by the gods. His followers hope to set him free. Despite his imprisonment, Tharizdun can still grant spells to the mad cultists who work in his name.
 
With his goal of destruction, Tharizdun is considered Chaotic Evil. Historically, however, he has been considered Neutral Evil. No beings have encountered Tharizdun for over a millennium, and his alignment can only be inferred from that of his insane followers.
 

Description

Tharizdun's appearance is lost to time. None now alive have seen him, and the gods who sealed him away do not share their secrets.
 
Tharizdun is a whisper in the minds of his followers, an eye watching them from behind and beyond their senses. If a creature of Tharizdun incarnates in the world, it is a black void, a malevolent darkness that swallows all and grasps its enemies with loathsome tentacles.
 
Judging by his children, it can be inferred that Tharizdun has some demonic appearance, but what that would be is unknown.
 

Appearance, Manifestations

Unlike most deities of Oerth, who take humanoid shape, Tharizdun appears as a dark, amorphous form reminiscent of a sentient sphere of annihilation. Contact with the deity brings insanity and death, and no accurate surviving descriptions or depictions of him remain.
 

Relationships & History

Tharizdun is hated by nearly all other gods, who would gladly put aside their differences to stop the Dark God from escaping his demiplane prison.
 
Rao, god of light, is particularly opposed to Tharizdun. Rao's followers believe that the Crook of Rao was originally crafted by that god for the purpose of banishing Tharizdun's dark forces.
 
Boccob, god of magic, actively opposes Tharizdun and works to ensure that he remains imprisoned. The power of magic is very gradually waning in the world of Oerth, and Boccob suspects that Tharizdun is responsible.
 
Tharizdun has no allies among the gods. The evil Archomentals, five powerful beings believed to be his offspring, are possible allies in his plots.

 

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Tharizdun was a long-forgotten interloper god from before all existence. Tharizdun is older than the Universe and possibly all universes. He & Lord AO are the originals, wihereas AO was for creation for others, Tharizdun is for himself. He knows he can destroy all that exists so he has full control. During the Dawn War, he created the Abyss, an act for which he was imprisoned by the gods. His followers hoped to set him free.
 
This seems to be why the 333 Gems of Tharizdun are so important.
 

History

Only the evilest of scholars know parts of the history of the Chained God. The Book of Vile Darkness hints at his origins, but it has only part of the truth. What follows is based on that foul lore; any more is known only by the most ancient of gods.
 
Tharizdun incarnated with the other gods at the beginning of the universe. He traveled the formless voids, seeking great power. At some point, in the blackest heart of the universe, he found it.
 
What this power was, it is unknown. It is said that Tharizdun found the crawling chaos at the core of the cosmos, and from it extracted a single shard of pure Evil. Whatever the cause, Tharizdun gained unfathomable power, power so great that he surpassed all his godly brethren. But in the process, he lost his mind and his body, becoming a thing of madness and death.
 
Seeking to bring more horror to the universe, Tharizdun created the Abyss. It is said that he planted the shard of Evil in the fabric of reality, and that it burrowed within, drawing a yawning hole down with it. The demons that sprang forth were of shapes unknown to modern beings, but they must have resembled the most ancient demons known. Examples like the sibriex allow scholars to infer they were things of slime and scarred flesh, covered in mouths and grasping talons.
 
With his foul children, Tharizdun waged war against the other gods. But though he was mighty beyond any of their power, he could not stand up to their combined might.
 
With magic so deep and essential that it has ceased to be magic, the gods threw Tharizdun out of the world, binding him with chains of great strength. But they could not destroy the Abyss; the darkness within the universe accepted it, and it became a plane, like the others that were coalescing at the time.
 
Through his children, Tharizdun spreads his cult. His worshipers give their lives to him, granting him energy that he uses to struggle against his eternal bonds. One day, he will break free.