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.
The game suggests that it was Ostrit and not their mother who cast the curse, as evidence by his journal.
In the graphic novels Edit
She did not appear in person, but Foltest speaks to her portrait at the start of Polish comics Geralt illustrated by Boguslaw Polch. She is also shown with first issue of The Witcher: Curse of Crows when Geralt retells Ciri the backstory of his first striga contract.