The site of ceremonial sacrifices, a great slaughter, or some other calamity, the spirit of death hovers over blood rock.
Effect: A creature standing in a square of blood rock can score a critical hit on a natural die roll of 19 or 20.
Usage: Blood rock makes encounters slightly more unpredictable, which usually favors the monsters. You can use a large expanse of it to make everyone score critical hits more often, or use just a few squares of it and let the adventurers and monsters fight over who gets to stand on it.
Published in Dungeon Master's Kit, page(s) 209, Dungeon Master's Guide, page(s) 67.