One of the things a lot of people seem have problems with is where and how to get ideas. Personally, I never found this to be a problem – and one of the best sources for plot ideas is still the real world. This is one reason among several why I read the news almost daily.
Here’s a very good example, posted on the BBC News Site. In Indonesia, a man was executed for the murder of over 40 women ten years ago:
An Indonesian man who murdered 42 women and girls in black magic rituals has been executed by firing squad.
Self-confessed “shaman” Ahmad Suraji, 57, told police he killed the women to improve his magical powers.
He had been sentenced to death in 1998 after police found the women’s bodies buried in a field in North Sumatra.
Suraji’s victims had come to him for supernatural help with their finances and love life.
Police said he persuaded them to be buried naked up to the neck before strangling them.
This is a pretty obvious plot idea, and it can be transplanted to pretty much any setting and genre. In a horror setting or a fantasy world, the man may actually possess magic powers, but even in a hyper-modern science fiction setting there will always be people who believe in the supernatural and there will always be con artists (and the honest insane) who take advantage of this.
The protagonists could be involved in tracking down this serial killer, or they or a friend could be persons who ask the “magician” for help. The killer might actually be using the victims to prepare a grand ritual to summon a supernatural being straight out of a Lovecraft novella. They could be explorers or tourists in the Caribbean, and stumble across a witch doctor who buries his victims in an attempt to turn them into zombies.
For extra color, change the method of killing – Instead of strangling them, he could use a ritual dagger and slowly bleed them to death. Or he could leave them, helplessly buried to their necks, when a nasty monster approaches out of the darkness and eats the victims alive.
Of course, it is just as possible in your story that the “magician” is an innocent man, himself the victim of a witch-hunt, and the players must rescue him before he is executed by the firing squad – maybe because it’s “the right thing to do”, or because they were hired (or blackmailed) to do so, or he may actually possess some vital knowledge the characters need in your campaign: A rare spell, knowledge of the whereabouts of the sunken treasure, plans to the secret underground bunker the terrorists hide their improvised nukes in… You get the idea.