The Witcher Potions - Black Blood
Turns the imbiber's blood into poison; blood becomes lethal to monsters which drink it.
The potion is made by mixing ingredients providing a total of three measures of vitriol, and one measure each of rebis and aether; strong, top quality alcohol must be used as a base.
In the distant past, mages working with witchers developed this potion specifically for use in fighting cemetery and crypt dwellers that drink the blood or eat the innards of their still living victims. Witchers use Black Blood unwillingly because the potion only works when a monster begins to feast on their body. Their transmuted blood proves to be a deadly drink.