From Dark and Darker Wiki
Technically, the probability of obtaining Cursed Crown from The Forgotten Castle's 'Crypt' map from the 'Inferno' is 1.65% P(Ghost King) = 1/3 = 0.33 ~ 33% P(Cursed Crown) = 1/50 = 0.05 ~ 5% P(Ghost King and Cursed Crown) = 0.33 × 0.05 = 0.0165 ~ .1.65%