A blood grove is formed when a tree grows out of an indeterminate amount of corpses, is in an area with a large amount of negative energy to the point where they rarely form naturally outside of warzones, that combined with the stigma against necromancy makes documentation extremly difficult due to any new groves being quickly eradicated by the surrounding populace, not to mention the danger in approching one. - A brief note in The monster compendium third edition.