Belladonna is a toxic plant also known as "deadly nightshade" whose berries are used in the manufacture of cosmetics, poisons and sometimes alcohol. Regis was known to spike his moonshine with Belladonna. Also translated as "banewort" or "banewart".
“
The witcher smiled wryly — he knew what he looked like. After drinking a mixture of banewart, monk's hood and eyebright the face takes on the colour of chalk, and the pupils fill the entire iris. But the mixture enables one to see in the deepest darkness and this is what Geralt wanted.
”
— pg(s). 22, "The Witcher", in the collection The Last Wish (US edition)