I agree, eating souls as a warlock deal.
I also like it as a theme for fae patrons.
I have a warlock whose patron is the fae lord of cats. It's an unhealthy relationship where he was her only significant relationship growing up. She grew up in an orphanage as the only tiefling in a place that viewed infernal blood as a curse or at the very least a bad sign, something wrong with her.
She met her patron as a child. On the way home from the library, she came across a group of older boys, teenagers twice her size, who had a bag of kittens. The gang was discussing drowning them or lighting their tails on fire for sport. She told them to let them go, and was willing to get in a fight to save the kittens.
Her patron came to her afterward in the form of a black cat with a white tuft on his chest and spoke to her. He offered her a boon for what she'd done.
She asked to keep him, he said he could not be kept, she asked him to be her friend, he agreed to be her friend until she came of age. So he appeared to her as a boy her age or as a cat. (very Peter Pan and Wendy meets The Cat Returns and Fruit's Basket vibes). They snuck out from the orphanage at night.
When she came of age, he asked to keep her, giving her a collar with a bell as a symbol of the pact (Pact of the Talisman). He only cares about his own people, cats, and has a cat's morality. She has self esteem issues and trust issues, so despite their friendship and her service protecting and helping cats and punishing anyone who hurts cats, she believes that this deal is means her soul belongs to him and he will eat her soul when she dies.
She asked him about it.
"If I say yes, I’ll be yours?" she asked the beautiful boy playfully crouched on the roof. He seemed almost like an elf but not quite, too tall, too beautiful, scelera eyes still like a cat, and long fae ears still covered in black fur. As always, when he took this humanoid form, he looked the same age as her, though she knew well enough he was something ageless, immortal.
"Forever," he agreed, his grin wide and full of sharp teeth.
Her heart skipped a beat. She had thought that when their deal ended, when she came of age, he'd leave. Vanish like an imaginary friend of childhood. Leave her to get along on her own now, since she was grown, and he'd taught her enough to take care of herself on the street. Sneaking, stealing, and how to make the rooftops and labyrinth of alleyways a thoroughfare.
Yeah, she could land on her feet, but she didn't want to lose him. She looked at the gift he'd offered her, a collar with an oversized bell. Her mind replayed his offer 'I want to keep you, but only if you agree'
Of course, she agreed. No one had ever wanted her. She wanted to be kept, to stay with him, to be his...but there had to be a catch, right? She'd read about deals like this. He got something out of this.
"So what, when I die you get my soul?" her voice all bravado, casual and tough, as if she was scoffing at the offer. Even though she did not want to hesitate.
"Yes, I will take it from your lips with your last breath," he said, "but until then, I'll be with you and teach you more tricks and show you so many things and you'll help me and my people as needed."
"They say cats will steal your breath when you sleep, and eat the souls of the dead. So I'm yours for life and then -"
He laughed and smiled at her wickedly. "Are you scared? Here I thought you were fearless. Always willing to leap, whether from the roof or into trouble."
"Trust that you land on your feet," she repeated words he'd told her so many times, "but trust nothing else. Be alert, be cunning, be bold but remember the world is full of teeth and claws stronger than yours, so be quicker and be clever."
"You remember well," he praised.
"And I remember that you don't care for mortals, or morals, you're concern is your kind and I am not a cat," she said, "I'm someone you owed a boon paid with companionship for seven years but that's done. Now you want to keep me?"
"I've grown fond of you," he admitted, "you're not just any mortal, you're mine - if you still want to be, hence the offer. You know I cannot tell lies."
"You haven't said you don't plan to eat me," she pointed out.
"I don't plan anything," he said, "not that far ahead. I didn't plan to offer this. I didn't plan to grow... attached and intrigued at what you'll do next and what you can become. I do not know if I will grow bored or roam or lose interest in you in the future, but regardless you can have my gifts and keep what you learn but there's always a price, and I don't know what I'll do with you forever, what may amuse me."
She could picture him playing with her like a mouse or a bird, putting off eating her but doing it in the end. The world is full of teeth and claws, and if this being ever wanted to catch her, she would be helpless.
He was the lord of a Kingdom in Faerie, a member of no court and welcomed in all. He sat on the throne of the Summer Court and in the lap of Queen Mab herself, belonging to no one but welcome by all, or at least not kept out, for that would be futile, this Archfey Lord of Cats.
She knew that mortals should be wary of deals made with fae. She knew he would not promise to never hurt her, as a cat he could do from purring to biting when he was done with pets or biting just to play. Fae were changeable, and after a lifetime, how useful would she be to him if cats or cat faeries really did have a taste for souls, lapping up people like cream?
He stood and sauntered towards her, holding out his hand for the collar. "It's just an offer," he said, "I'd like to keep you but I'd never fault anyone who did not want a collar, my little Ferral Vad." He spoke the name she'd chosen for herself, one based on defining how she saw herself, what she was, as was Tiefling custom. Fair for short, which she thought funny.
"I want you to keep me," she said, because, regardless of anything else, including well-founded concerns about this sort of arrangement, she did want him to stay, and she liked the idea of being his. She turned, using a hand to pull her short cut indigo hair out of the way to expose the back of her neck.
She placed the golden collar against the blue black skin of her neck. "Will you it put on for me?"