Thuriel

Thuriel was a so-called rogue devil who sought to supplant the strict hierarchy of Hell and use alternative means to increase his own power.

Sunder's Downfall
Thuriel, like many devils, was keen on tempting mortals with lawful but underhanded contracts. According the legends, he was able to tempt a powerful monk named Something by convincing him the gods were nothing but parasites of mortals and should be opposed. So, the monk signed a contract offered by Thuriel, granting him devilish powers in order to fight the gods themselves. In return, he owed the devil his soul.

His powers and life extended by Thuriel, the monk was able to round together a group of like-minded and powerful individuals. He was also able to inspire armies of mortal men to his cause, who started at first to tear down and destroy the temples of their gods. Soon a full conflict erupted between the faithful and the faithless. Clerics, paladins, and armies of mortal followers opposed the monk, but he was able to defeat them. Angels and other celestials were summoned to the plane but were inevitably defeated by the monk and his powerful allies. It took nearly a century of bloody war, but finally the gods decided to abandon the plane entirely.

This has the unexpected result of driving the plane into stagnation. Without the gods, the natural cycles of the world ended. There were no more stars in the sky. The plane became locked in either total darkness, twilight, or burning daylight depending on your position. This was the Downfall of Sunder.

Return of the Gods
Sunder remained a dark and horrible place for its mortal survivors. Using his great powers, the monk and his allies did their best to build a new society, and offered to craft an entirely new demiplane in which the people could one day inhabit and escape the dark. However, the monk realized should he ever leave this special demiplane, that Thuriel would be there to claim his soul.

Fearful, the monk sealed himself away, and the promised "new world" never came. The people continued to live their lives, adapting as they could, uncertain what would happen next. Thuriel became frustrated. He had offered much to the monk in order to achieve this victory. He had made contracts of his own with other devils who expected a return on their investments. Desperate, Thuriel sought a way to breach into Sunder itself and retrieve the monk any way he could.

Over those many years, Thuriel discovered that the shadowed side of Sunder had drifted closer to the Abyss, which was granting access to it by weak shadow demons. Using the same gateway, Thuriel was able to enter Sunder and found before him the empty thrones of the gods, their domains abandoned. He took up one of these thrones, a difficult process for a mere devil, but over time adjusted to the position.

As he did, a star re-appeared in the black night sky. People looked upon this star with wonder, with some praying to it in hopes it was a returning god. This only helped empower Thuriel and allow him to better fulfill the role as a fledgling deity. Over time, he inspired clerics and paladins to his cause, tempting and deceiving them into helping him.

and other things happened