Tharizdun

Tharizdun, sometimes called the Chained Oblivion, the Undoer, and the Father of Wickedness, is the Deity of Entropy and Secrets. In the days of the Dawn War , It did not ally with the other Gods against the Primordials in direct conflict. Instead, It professed to be seeking secret ways to undermine and destroy them using a Seed of pure evil. Some tales say that Tharizdun made the Seed, while others say It found the Seed in some dark and desolate corner of the universe and was studying it. What is generally agreed upon is that Tharizdun took the Seed to the “bottom” of the Elemental Chaos and planted it. Its evil slowly grew, consuming the plane itself, Elementals, and Primordials alike before its advances could be halted. The new, twisted plane that had been created was now the Abyss, and those elementals and Primordials that had fallen victim to the Seed were now the first Demons.
(Some versions of the story say that some of the darkest and most powerful demons, including Demogorgon, were not born of Primordials, but formed from the stuff of the Seed itself. Other legends say that, when Demogorgon plumbed the seas of the Abyss, he found Dagon there, already waiting. The truth of such tales is not known.)
For Its crimes, Tharizdun found both Gods and Primordials allying against it to bring it to justice. It was captured, chained, and imprisoned in the Astral Sea in the shadow of Tol’s moon, Kassyle. In order to prevent its hidden prison from being uncovered, Tharizdun’s name was stripped from the memory of all living things, and It became known as the Nameless, a dreadful something outside of space and time.
Over millennia, Tharizdun strained against the walls of Its prison, but to no avail. In time, however, something opened just a crack in Its featureless horizon. Some scholars believe it was The Rending  that caused the crack, or perhaps Tharizdun caused The Rending when it cracked the seal on Its bonds. In any case, It was now able to whisper into Tol. Over time, It built an identity as the Elder Elemental Eye to allay the fears of Gods who might’ve thought the Chained Oblivion was taking steps to escape.
In time, It found a willing ally in the Summerling lord, Kaldakos Greysails. Kaldakos formed a mad plan to use Tharizdun to get back at the entire world that had condemned him in his youth and killed those he’d loved. Kaldakos first founded a group of other powerful beings that he called the Five Who Call. He pitched to them the idea of power and revenge on various forces that were arrayed in the Seven Kingdoms. At the same time, he cultivated a public guise as a humanitarian and devout follower of the Gods. In this guise, he supported a group called Those Who Answer, members of the Seven Kingdoms and elsewhere who wanted to help others.
In secret, Kaldakos also formed a cult around the Elder Elemental Eye, gathering various folk who served the Primordials for his purposes. Among these people, he became known as the Grey One. They knew he had been punished in his youth as a worshiper of Yan-C-Bin, so it was easy for them to believe he still served the Primordials.
All of these forces acted as the linchpin for Kaldakos’ plans – to release the Tarrasque - and, because of the way he did so, to make every life it took into part of a sacrifice dedicated to Tharizdun. With each death it caused, the Tarrasque brought Tharizdun closer and closer to freedom. Even Kaldakos’ death at the hands of the Fire Wasps seems to have ended up dedicated to this purpose.
Now, with the Tarrasque on the loose, it remains to be seen if Tol is about to have a dark god unleashed on it. Tharizdun was able to partially shatter Kassyle, but he remains imprisoned…for now.