The Raven Queen

The goddess of death and fate and the warden of the Shadowfell goes by many names. To the humans of the Empire she is the Crone Moregh, the Black Lady. To the Delvers she is the Matron of the Last Hearth. To the orcs and goblinkin of the north, she is She That Stalks. Elvendom know her as the Raven Queen, but none living or dead know her true name, as this was lost when the portal to the Shadowfell was opened and the Raven Queen ascended to godhood.
In the aftermath of the War of the Dawn, there was a time of wild magic. The appearance of the Fey had opened the floodgates for magic to once again flow through the lands of Dyrn, which had been bereft of magic since the end of the Dragonform War and the formation of the celestial scale. But that magic had nowhere to go; the ancient leylines of magical power remained obstructed, and as the magical power of the feywild gathered within the land, it began to pool, forming areas of wild magic where the natural laws of the world were broken, where the dead walked, and where mages, if they were not very careful, could lose control of their powers in vast, destructive displays.
The Paladin Prince Iandric, guided by the visions of an unnamed sorceress, set out on a quest to try and save the world from this peril. Harried by the drow, whose new goddess Lloth hoped to harness the power of wild magic to blot out the sun and grant her new followers dominion over Dyrn, Iandric and his crew sailed the seas to eventually find the isle of Fel Eris, atop which lay the ancient and broken Necromancer’s Tower. As his crew fought off their Drow assailants, Iandric escorted the sorceress to the top of the broken stair, where he died thrice, fulfilling an ancient prophecy and opening a portal to a new realm, a nascent Shadowfell.
The powers invoked in this event were vast. Like a circuit being completed, suddenly a century of wild magic was able to flow through the leylines of creation once more. The sorceress acted as a conduit, guiding and shaping these energies through the portal to form the Shadowfell, and in doing so her mortal form was destroyed. The resulting outpouring of energy also claimed the lives of the elven and drow crews that had accompanied them to Fel Eris.
On the other side of the portal, the sorceress arose, and began to tread the endless stair. With each step, she shed more and more of her mortality, until finally, after a time unending, she crested the stairs, blind and alone. As she did so, a raven cawed, flying down to perch on her shoulder, and through its eyes she was able to look over her new domain.
It is said that the Yrdragon itself spoke to the Raven Queen and told her of his plans for the universe, so that she may ensure that the fate they had planned would come to pass. It is also said that the Raven Queen is just one facet of a god of many faces, and that every sphere, every world has its own story of the ascent of a mortal to godhood and dominion over death and fate.
The faithful crew that died trying to defend Iandric and the Sorceress were reborn in the Shadowfell as well, and became the first Shadar Kai, elves of twilight that are now indigent to the Shadowfell and remain loyal servants of the Raven Queen. The drow who pursued them were also drawn through the portal, but as they tried to climb the endless stair, they became twisted and corrupted, eventually becoming the Greater Nagpas.
The domains of the Raven Queen are death and fate. Few worship her, but all turn to her at the end. All souls, when they have left their mortal forms, travel along the leylines to the Shadowfell, where they must tread the endless stair. As they climb, they are judged; they face many trials at the hands of psychopomps such as angels, demons, and the remaining nagapas, and depending on the outcome of those trials, their fate is decided. Those who have the claim of a deity on their soul and pass their trials are released from the stair and sent to the celestial plane of their deity. Those who have transgressed against the Gods or fail their trial fall and are sent to the lower planes to sit out their eternal punishment. And some, whose fate is not yet sealed, are sent back to the material plane to be reborn, until fate is done with them.
Only the most mighty souls, either good or evil, make it to the very top of the stair. Here in the Chamber of Memoriam, the Raven Queen sits, eyes bound in the fabric of the Shadowfell itself. She herself casts judgement on those souls who have survived the many trials of the stair. She is guided in her judgements by the three Muses, who assist her in weaving the tapestry of fate.