Initial thoughts. Five-sided structure, larger than a boxing ring but similar in construction. Five ropes linking cornerposts, lowest at approx ankle height, highest at nine feet above surface.
Currently has PVC floor, marked with following symbols - sumo ring with pentagram inside it. Outside that, perimeter of greco-roman wrestling ring. Outside that, outline of standard boxing ring. All features in very dark red, almost indistinguishable from black by five days from flooring's installation. Any human or parahuman blood spilled on the PVC flooring immediately soaks through without marking the surface. The flooring may be replaced at any time.
The ring is a method of feeding blood into a cthonic gateway, which uses the ring floor as a door. Minor gruesome effects take place in the vicinity of the ring after five days with no blood spilled into it, or after 25 days without anyone dying within it. These increase over time until, after 25 days with no blood or 125 days without a death, something starts to come through. Have details of something terrible stretching the fabric of the ring floor, prevented from emerging by quick-thinking operative with a knife to hand. (?)
Initially fed by using as a D-class termination ground. Naturally at some point someone fought back, successfully killed guard - at which point minor gruesome effects went away. Fights permitted following this. Strange discovery - anyone who survives five fights gains terrible zen calm and no longer wishes to participate. Instead, at first blood shed within the ring on any given day, survivors simply appear at ringside and watch impassively. Four such individuals exist today. Suggest that this number is maintained by any means necessary. As soon as this discovery was made, all autonomous and/or self-aware SCPs were forbidden from approaching the ring, and it was quickly moved and set up again in a new site which contained no such entities.
Grade 4 Supervisor bollocks staff for recording bouts. Dr Bright or other G5 countermands this, saying that in current financial situation, revenues from sales in Russia, United Kingdom and Australia vital.