Very little is known about this dark, ancient evil. Very few arcane circles even dare mention his name, and even fewer believe the cult even continues to exist.
Tharizdun is the creator of the Abyss; who drove a shard of Evil into the Elemental Chaos. The Eldritch Beings, fearing the capabilities of Tharizdun's power for destruction, took this as a warning and acted upon him for his actions. Seeing that his mind was dark, bent, and willing to undo Creation, they bound him to the very Plane he spawned.
Some believe that in the deepest, darkest, most treacherous depths of The Abyss he lies shackled. Others believe the Abyss is Tharizdun.
The Eye of Tharizdun
Everyone knows of Sehanine, the Moon and the Goddess it's named after. What most don't know is there is actually a second moon, unseen to mortal eyes and only noticeable to those powerful arcanists. The Eladrin named this moon "Thariz-Shul", although most forgot the origins of "Thariz's Eye".
The ancient records that the Eladrin have kept over this mysterious moon is varied and contradicting. What is agreed is that very rarely does it Wax and reveal itself to the world, and when it does the world is usually undergoing a massive change. The last documented time of this was during the final days of the Dragon War after the establishment of the Empire.
Some schools believe that Thariz-Shul waxes during times of great magic and energies of Chaos and the Feywilds spill over into the mortal realms. It is time where the Raging Wilds expand and warp wildly, even more than usual, and the barriest between the Supernal and the Prime worlds grow thin.
Others feel there is a more sinister purpose. Those who catch on the the "Thariz-" origins of the name take note that these times are usually very dark; times where the forces of Law, Order, and Civilization have almost fell to certain doom. It was said the Betrayal of Bahamut was on the night of the Second Moon.
The barriers to the Supernal realms may become thinner, but those who've studied the cosmos know that Terra lies firmly above The Abyss, and this is a time where the armies of hell manage to break through the barriers.