Bheur Hags are known for their horrific and awful tendencies to cause distress. A Grandmother Hag, will do so to absolutely horrendous measure. This is the entity in which your life has been promised, but for now you enjoy the gifts in which "Granny" has given you.

Granny's Graystaff

You gain a graystaff. The graystaff is incredibly difficult to make and takes 1 year and 1 day. Thankfully, this graystaff was so generously gifted to you by Granny. It acts as your spellcasting focus, and it can only be used by you or another of Granny's "Promised". While wielding Grannys Graystaff you gain special bonuses and access to certain spells. Add the graystaff to your inventory. (can be found in my homebrew creations)

If you lose the graystaff, you must spend an hour performing a ritual in which you entreat Granny to get it back to you. Uh-oh!

"My my my, oh whatever shall we do with you. Tsk, tsk, tsk." - Granny Bheur

Creeping Cold

At 1st level, Granny gifts you with the knowledge of cold environments, you gain advantage on saving throws and ability checks to traverse ice or snow, and difficult terrain made by ice or snow does not cost you extra movement.

Additionally, you have resistance to cold damage.

Chilling Horrors

Granny Bheur grants you the ability to create an area of whiteout conditions, and creatures who find themselves within find their minds infested with illusory horrors.

Starting at 6th level, once per short or long rest, as an action you create a whirling storm of sleet and snow. The storm has a radius of 30 feet and moves with you, the area is considered heavily obscured and difficult terrain. The storm lasts for 1 minute, or until you dismiss it.

When you activate this ability, you may select a number of creatures up to your Constitution modifier. Those creatures,  yourself included automatically succeed the Wisdom saving throw. Any other creature that enters or starts its turn in the area must make a Wisdom saving throw. On a fail, the creature becomes frightened for 1 minute as its senses are assaulted with grotesque images, sounds, and smells. While frightened in this way, a creature is incapacitated, can’t understand what others say, can’t read, and speaks only in gibberish. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.  If a creature’s saving throw is successful or the effect ends for it, the creature is immune to this effect for 24 hours.

 

Sustenance From Suffering

Starting at 10th level you,  when a creature you can see dies, you can use your reaction to gain temporary hit points equal to your warlock level plus your charisma modifier. You can do this a number of times per long rest equal to your Charisma modifier +1.

Additionally, when a creature becomes frightened by you, allies deal an additional 1d4 psychic damage with weapon attacks or spell attacks against that creature. When the frightened condition ends, this effect also ends.

(the spell or weapon attack must deal damage to trigger the 1d4 additional damage)

Bitter Hearth

Starting at 14th level, you can use an action to create an icy door to your lair. You determine how the door looks and its size. The door leads to an icy plane with a cave entrance some hundred feet away in a place of your choosing. The area is perpetually battered with a bone chilling snowstorm that makes the entire area heavily obscured. The snowstorm cannot be dispelled, and only you can see through it as if its only lightly obscured. The area around the cave stretches on without end and the snowstorm never ceases. Inside the cave is your lair, you determine its dimensions and size although it cannot exceed the area within a 50 ft radius. You can furnish and stock the cave how you see fit, but the items are not conjured and you must possess them. The door must remain while you are in your lair and creatures can enter, however you know the general location of creatures within your lair. And Granny knows too, Granny always knows.

Additionally, you are now immune to cold damage.