I think in this video the grim reaper is actually hanging below the quad copter by fishing line. I think the liability from a crash would be too high to mess around with.
You are probably referring to thisMatthew's right--there's even a video on how it was done. I did see another video where someone built a ghost directly on a quad copter and flew it up and down a residential street.
That's the one! Thanks for finding it.You are probably referring to this