The Plain of Blood was an expanse within Vulcan's Forge on Vulcan.
According to some, Surak crossed the expanse when the hot blood of battle still flowed green, but with logic he cooled it. (ENT: "The Forge")
Jonathan Archer and T'Pol, led by Syrran, came across the Plain of Blood while searching for the Syrrannites in 2154. Syrran conveyed the history of the region to the others. (ENT: "The Forge")
In the script of "The Forge", the Plain of Blood was described as "an expanse of brutally rough terrain, dusted with pockets of sand, shimmering with intense heat distortion."