Does it have to be an individual fey? Could it be an entire coven? Could be interesting to have your warlock be the charge of, say, the Hourglass Coven. They each distrust each other and want to get one up on the others all the time, so they could each be using you to fuck with the other two all the time, essentially giving you carte blanche to do as you want as long as you are constantly a thorn in the side of all 3.

Could even flavor your spells to take on the aspects/traits of one of the three, like that individual hag has given you that specific spell. Divination and control magic from Endelyn with something like suggestion or hold person controllingthrough puppet strings. Skabatha can give you summoning spells and enlarge/reduce with all effects appearing like wood carved toys. Bavlorna gives you all necrotic magic and polymorph with the forms they take looking like stitched together Frankenstein-esque monstrosities.

Or maybe your Patron is specifically Baba Yaga, but she tasks you with being the Hourgalss Coven’s errand boy while also being her eyes and ears on her daughters’ activities.