Clownoid

A large clown toy, similar to that scary one in Poltergeist. Contains sophisticated robotics that enable simple movement and speech. Give it to your victim (perhaps a naughty child?) as a gift, and let it lie dormant for a couple of weeks.

Inbuilt sensors will detect suitable conditions (lightning storm, victim is alone etc), and the Clownoid will begin to do its stuff. Beginning with subtle changes in position made when the victim is not looking, it will create the tiniest wisps of doubt and fear in their mind. Next step is to make a strange noise that is just audible to the victim when they are out of the room, so that they come to investigate. This is repeated a couple of times.

Suddenly the Clownoid's eyes light up and it makes a kind of clown laughing sound. This will make the victim jump a bit, but the fear will subside and they will go to it to investigate, thinking "wow, i didn't know it made noises".

Showtime.

Suddenly, as they are looking deep into the clown's eyes everything seems to happen in slow motion. The clown's arms come up quickly to lock around the neck. The legs start kicking. The eyes widen. But worst of all is the voice - deep, scary and screaming, it unleashes a personalised wave of fury and evil, using the victim's name of course, and also saying things that have been carefully selected by the pesky prankster to have the most effect.

As a final touch, the Clownoid would suddenly revert to a limp, lifeless toy, and the victim's family have the option of continuing with the prank by refusing to believe the child and not allowing it to get rid of the thing.

This could be accomplished with existing technology.