The Ring of Winter

Shadowfell
The Ring of Winter was created in the Shadowfell, specifically in the shadow of Faerun, by a group of Shadar-kai working under command of The Raven Queen, the ruler of one of many Domains of Dread, and considered a god in Faerun, Nentir Vale. (This Raven Queen is not to be confused with the Matron of Ravens of Exandria, nor Morrigan the Raven of the Sickle.)

Through the creation and use of this ring, the Raven Queen took the domain of Winter for her own from Auril, the previous goddess of winter.

The Ring was eventually stolen from the Raven Queen, and its location was unknown for many centuries.

Temple of Ubtao
Eventually, the ring came into the possession of Dhalmass Rayburton of Cormyr, who mistook the ring for a normal magic item to be used. The ring's corrupting influence caused Dhalmass to murder several people, and then he sealed it within the Temple of Ubtao in Mezro.

The former Harper, Artus Cimber, spent his life searching for the Ring, eventually finding it in Mezro. He kept the ring for nearly a hundred years.

The Astral Serpent
The Heroes of Phandelver encountered a group of Frost Giants searching for the ring, led by Drufi. They were attempting to use the ring to initiate an Eternal Winter throughout Toril. Drufi was slain by Jericho, and the party learned from Sirac, Artus' grandson, that the Ring of Winter was still in Artus' possession, who was at that time still in Chult.

After the death of the Astral Serpent, when many worlds temporarily collided, the area of Chult was duplicated in the world of The Sickle, and Artus, along with the ring, was translated to this new world, unbeknownst to him.

Tomb of Annihilation
Violet, Uboth, and Nailo encountered Artus Cimber multiple times on their search for the Lost City of Omu. The Black Network, the Reapers, more frost giants, and Red Wizards all searched the jungles for the Ring, along with Galanthus, a paladin of Auril, the fallen goddess who was also translated from Faerun to the Sickle.

When Artus was caught in the middle of both the Black Network and the Red Wizards, he unwittingly allowed the Ring to take him over in order to defeat them and survive. He managed to win the fight, but in the process, Artus was lost to the Ring.

After the Death Curse was cured by Violet, Uboth, and Nailo, they encountered Galanthus, who recruited them to help him defeat Artus Cimber and claim the Ring. As the ring attempted to take over Galanthus, Auril imposed her will over the ring itself, hijacking its mind-control powers, and turning Galanthus into her thrall.

The Red Hand
With Galanthus and the Ring fully under her control, Auril recovered the body of Ingrid and returned to the Sickle. Here, she used her slowly returning godly powers, in conjunction with the Ring, to build an army of thralls. She branded them with the hand wearing the Ring, leaving a red hand print. It was due to this brand that the army came to be known as The Red Hand.

Auril's plan was to take over all of the Sickle using her army, and then impose herself as the one true goddess, and finally reascend to godhood. If she succeeded, she could return to her world and challenge to Raven Queen.