Witch offers cash for kids.
Sacrifices them to the portal to hell in her oven.
The sacrifices keep the portal closed.
People hate her, so she needs to keep herself safe:
* The forest around her cottage is enchanted. Anyone who talks about violence will alert the trees. They will start to move, shifting the paths and trails so that angry villagers get lost and eventually give up and go home. Looking at the stars/moon/sun (2 in 6 chance it's cloudy, tough luck) will reveal that the PCs are being led around; otherwise, the PCs will wander about for 1d4+2 hours before noticing anything. DMs should note that PCs trained in wilderness survival will realize they are wandering around in circles.
* Gnomes live in the woods. They have no problem with the witch - they know what she's up to. They are shy and fearful, so are reluctant to speak with the PCs. They can be coaxed out of their homes (under the roots of trees) by shiny, sparkling gems (100 GP+), song, or dance (roll under Cha/Dex or DC 15).
* The witch has an intelligent cat familiar. It can move through shadow (line-of-sight). It roams around the forest, keeping an eye on things. It can talk. It has a fondness for milk.
* The witch is a 5th-level magic user who can:
- Bewitch you with a look, causing you to be overwhelmed with a sense of peace and good feelings. If you resist this spell, you're immune to future attempts. People under this effect wander about aimlessly; the witch guides them away from her cottage once subdued. She must look you directly in the eye to do this, but she doesn't need any fancy words or hand movements.
- She fights with a curved dagger that has a wicked edge. When cut by the dagger, you are overwhelmed with ravenous hunger, and must eat any available food at hand. If no food is at hand, you can act normally, though hunger pains wrack your body.
- She can cause an open flame to give off a heavy, dark smoke. The smoke pours out at an extremely quick rate, filling a 20' cube in a matter of seconds. None but the witch can breathe in the smoke, nor can any but her see more than a foot away within its inky darkness.
- She is a horribly ugly hag. She was once a normal woman but she made a deal with a devil to feed this portal lest the hordes of hell run loose upon the world. This pact gave her immortality (and her other powers), but her body is ageing normally. Any character with a short life span (so not elves or dwarves) who can't help but think of the inevitability of death when looking at her (save to resist) suffer a penalty to attack rolls (-2) because of this.
If the witch is killed, a powerful devil will come forth. He will offer the PCs the same deal that he offered the witch: powers and immortality for children. Otherwise hell will break loose. The devil has a weakness: he cannot stand the sight of bees. If bees are presented openly to the devil, he loses all power. A nearby sage/library contains this information, as well as a method for closing the portal.