The Hexblade is a popular choice at my table, so I've been asked to come up with different Shadowfell patrons over the years. Here's the three that made it into my campaign; maybe I'll post more later.
MASK
The Master of Thieves, The Lord of Shadows
Alignment: Chaotic Neutral
Weapon: Longsword
Familiar: a tarantula
Tome: a black leather book with black pages, the words within scribed in white ink
Mask fancies himself to be the god of shadows, crime, rogues, and thievery. While he is not a god, he is nonetheless a very powerful entity worshipped by thieves, assassins, bodysnatchers, and other mortals of ill repute. He is known for his constant scheming, his cool head, and his hatred of rivals--particularly the god Olidammara. When he wishes to be seen, he appears as a thin humanoid dressed in black leather armor, wearing a black velvet mask and carrying dual longswords. He makes his presence known through the flickering movement of shadows and the sound of footsteps in empty rooms.
NERA, THE RAVEN QUEEN
The Queen of Death and Winter
Alignment: Lawful Neutral
Weapon: war pick
Familiar: a raven, obviously
Tome: a black leather book with a raven embossed on the front
The Raven Queen was once a powerful mortal sorceress and queen named Nera. When she died, her soul came to Pluton (the realm of Nerull, the god of death). Impressed with her beauty and ambition, Nerull gave her form and enslaved her to be his wife. She eventually stole Nerull's power over shadows and specters, and used that power to overwhelm him and escape to the Shadowfell. There she waits, and plots her revenge. She makes her presence known to mortals through black feathers, gusts of cold wind, and the sound of crows.
SHAR, WEAVER OF SHADOW
Mistress of Night, Lady of Loss
Alignment: Neutral Evil
Weapon: a quarterstaff
Familiar: a bat
Tome: a matte black scroll with a wood spindle, bound with a purple ribbon
Shar is one of the primordial entities that existed long before the creation of our world. In the chaos of the universe's creation, Shar pulled the darkness away from the light and wove it together to create the Shadowfell, the first shadows, and all incorporeal undead. It is said she can see everything that lay in darkness, and everything that ever happened in the dark. Thus, she is associated with darkness, shadow, caverns, and secrets. She makes her presence known to mortals through the sound of bats, the echoes of dripping water, and the moaning of wind.