Unsurprisingly this is a boon (or favour) from a witch.
People got to witches for the same sort of things as they go to any other kind of worker - power, money, sex, revenge, healing and the finding of lost objects. Since witches tend to be less prone to abide by social conventions they are traditionally more likely to assist with less reputable requests.
Now, given that your actual witch is someone who has obtained their powers by an infernal pact, such things are unlikely to turn out well. Using magic to do evil usually has consequences anyway, and if you have gone to a witch to procure maleficium he or she will make damned sure that those consequences rebound on you and not them if at all possible - otherwise they may well be more prepared to assist with wickedness as it will often advance the agenda of their infernal patron.
And then there's the matter of payment - a witch may take payment in cash or kind like anyone else, but they and the powers they work for tend to charge higher and darker prices for their help, particularly when the client has little or no leverage to haggle with. Really stupid or desperate people pay with 'a favour in return' … which can turn out to be anything and can end up with the luckless client being forcibly transformed into a shape that serves the witches purpose and obliged to toil for a disproportionate length of time. This is a common source of Transformed animals. Payment is rarely dependant on the client getting the outcome they wanted - in fact not getting what you planned for is pretty much garunteed in some traditions.
Conversely, a witch may accept very simple terms of payment if magical items are to be had and some mythological heros are able to pay their way quite easily with artifacts picked up along the way that the witch just happens to want. This may also turn out badly.
In many ways this is something of a deal with the devil by proxy.
Game and Story Use
- Use when your PCs are in a fix: a witch can help them … for a price.
- If your PCs want magical help to do something antisocial, a witch may be their only hope.
- Likewise, if they are dealing with obscure dark magic, a witch may be the best source of information.
- Someone who has made a bad bargain with a witch and needs help getting out of it can make a good client for PCs.