I can't remember the name of this piece or the artist but it is a pretty famous.
It depicts this massive army of creatures/demons marching out of the gates of hell. There is a lot of black and red in the painting. The line of demons/tormented can be seen stretching from the background to the foreground of the painting, I believe they are marching to the right.
It's an amazing piece, I also seem to remember it being done by a famous 70's sci-fi artist.