From Dark and Darker Wiki
Technically, the probability of obtaining Ghostly Essence from The Forgotten Castle's 'Crypt' map from the 'Inferno' is .66% P(Ghost King) = 1/3 = 0.33 ~ 33% P(Ghostly Essence) = 2/100 = 0.2 ~ 2% P(Ghost King and Ghostly Essence) = 0.33 × 0.02 = 0.0066 ~ .66%