It's the invented script from the Cthulhu mythos by the gothic horror writer HP Lovecraft. Cthulhu slept at the bottom of the sea, before being awakened by man or something. I don't know, I haven't read it. That's what the big scary monster thing to the left of the script is.

It's a big part of geek culture, it made its way into D&D, and via that into everything else.