Morrigan

The Raven Queen is obsessed with collecting and observing mementos and fragments of memories and essences. Many sages speculate that she is in fact insane, believing her methods and behavior little more than aimless curiosity or a desperate attempt to prevent her fragile existence from fading into oblivion. Others maintained that there was a cosmological function that was performed by the Queen's actions, as she purifies souls by forcing mortals to deal with their fears and pains. It was believed that her true intentions were only known to the Lady of Masks.
One of the Raven Queen's divine realms in the Underneath is a castle known as the Fortress of Memories. It is a place of overwhelming sorrow, overflowing with memories taken from mortals and fragments of dead deities, material objects collected by her shadow-fey and brought to her as gifts, and apparitions of creatures and places associated with strongly emotional stories. Swarms of ravens occasionally flew out of the fortress, taking her cryptic messages to the far reaches of the multiverse and acting as the Queen's eyes and ears while away.
The whole existence of the Raven Queen is focused on collecting memories and strong emotions, typically associated with loss and tragedy. To that end, she dispatches shadar-kai to the far confines of the multiverse to wait for such an event to unfold, as scouted by her ravens. Then they collected mementos, such as trinkets, memories from the living and sometimes the souls of the dead, and brought them back to her.
The Raven Queen also claimed domain of the souls of all shadar-kai (shadow fey), which always returned to her after their deaths. She also claims the souls of her mortal followers, or at least those who accept her gifts. She also has dominion over The Gloaming, powerful fey spirits that have adopted the shadowy voidlands of the Underneath.
She was said to have forged the first weapons infused with shadow magic. These sentient blades, such as the Blackrazor, were found in many locations throughout the multiverse and were capable of binding with their wielders. For that reason, the Raven Queen was a popular hexblade patron among warlocks.
The Raven Queen is the sole god inhabitant of The Underneath, but it is believed that was not always the case. The Underneath follows a lot of the principles priests attribute to the Void. That said every other person in the Underneath in the current age is one of her followers, either shadow fey or mortal.
The Maiden of Masks (the god of the Astral Plane and ruler of the city of Sigil) is the sister of the Raven Queen. Both are considered neutral gods and stay away from the conflicts of the Upper and Lower Planes.