mark of the beast (See this explanation for further details)

Revelation 16: 2

2 And the first went, and poured out his vial upon the earth; and there fell a noisome and grievous sore upon the men which had the mark of the beast, and upon them which worshipped his image.

Revelation 13:15-17

15 And he had power to give life unto the image of the beast, that the image of the beast should both speak, and cause that as many as would not worship the image of the beast should be killed. 16 And he causeth all, both small and great, rich and poor, free and bond, to receive a mark in their right hand, or in their foreheads: 17 And that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name.