Adda and her brother had an incestuous relationship that resulted in a pregnancy. This child was cursed (presumably either by Ostrit, who fell in love with Adda, or by their mother, who was furious about the incestuous union between her son and daughter) and both mother and child died just after the birth. Foltest ordered that the two be buried in the Royal vault beneath his castle in Vizima.
Seven years later, their daughter came back to life - but as a striga. The curse was eventually lifted by the witcherGeralt of Rivia, and the young princess was named Adda, after her mother.