The Plain of Blood was a place within Vulcan's Forge, on Vulcan.
Vulcan legend states that Surak crossed the Plain when the hot blood of battle still flowed green, and 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")