Then use one shadar kai (the CR 7 one) and 2 elder darklings. You can further augment the difficulty by having the shadar kai summon lesser darklings

They pack a punch but are easily one shot, perfect for making a fight more lethal but less grueling

>Would they have the magical power to make that deal happen?
With a magic item? Yes.
>I thought the potions would be a good symbol or carrier for that magic.
Players are paranoid about con suming anything given to them, more so if it's a creature known for making deals, especially fey.
>They don't have to drink it, but it's drink it or die.
Choo choo mother fucker.
>I could just make them verbally agree and be bound by it then.
Give them power in exchange for their help, it's a far better experience than forcing them into a cut scene.