Each player sacrifices a creature. Create an X/X black Demon creature token with flying, where X is the total power of the creatures sacrificed this way.
5/29/2014 You choose which creature you are sacrificing first, then each other player in turn order does the same. The creatures are all sacrificed simultaneously.
5/29/2014 Use the powers of the creatures as they last existed on the battlefield to determine the value of X, even if they were negative. For example, if the sacrificed creatures were 3/3, 5/5, 0/4, and -2/2, the Demon will be 6/6.