The myth is located in a building in Liberty City. The building itself is not paranormal in any way, but there is an occasional titular blood stain, one of which is shaped like a particular rat head, leading players to believe it is a reference to Ratman. Critics have said, that this is motor oil, because of claims of Ratman's blood being motor oil instead of blood.
The true myth from this location is that NPCs inside the building are doing many odd activities. Players commonly see pedestrians inside the house vanish into thin air. Some even claim to see blood pools form where ever the pedestrian has vanished.
The house can cause disruptions on the multiplayer mode of GTA IV. A pedestrian will disappear on one player's screen whereas it can still be seen on the others'. NPCs who don't evaporate will either exit through a back door or walk in between some dumpsters in the courtyard and get stuck, or they will reach the roof and jump off. Sometimes the pedestrian will die, or they will enter the building and jump off again, leading to their demise. Most users pass this off as just a glitch of their walking patterns, similar to the Suicidal Photographers of GTA San Andreas, but there is a small group of myth hunters who truly believe that this is a supernatural phenomenon.