Birna Bran is the wife of King Bran of Skellige and the mother of Svanrige Tuirseach. Unlike her husband she has never been respected and her son was always considered weak-minded. She never respected the traditions of Skellige and saw the monarch system on Skellige as flawed and thought a king shouldn't be elected but that the line of the former king should inherit the throne. This clashed with the traditions of Skellige, where king's children don't inherit his power but the king's successor is chosen by nobles. Tp be chosen king, a candidate must have many fighting achievements and make lots of deals with nobles.
After the death of her husband King Bran, Birna once more showed her distaste for the tradition of Skellige. Normally, a king's wife would board the funeral boat with her deceased husband and would die with him when the boat was hit with a flaming arrow and turned into a pyre. Birna refused however. After the funeral, Birna plotted to have her son Svanrige elected the next king of Skellige. As Svanrige was considered weak, Birna planned to remove the other king candidates to the throne of Skellige. She intimidated a stuart of clan An Craite to poison drinks with a mixture that transforms drinkers into bears. That caused a massacre which eliminated competition for Svanrige to claim the throne. Blame for this fell Clan An Craite, as they were the host of the feast.
Geralt can help Cerys or Hjlamar in investigation of the massacre and reveal Birma was the mastermind behind the poisoning. Although she attempts to talk her way out of it, even her son Svanrige speaks against her, claiming that Birna told him to leave the hall shortly before the massacre. That convinces the jarls of Birna's guilt and they sentence her to die. Later Birna's body can be found chained to a rock on a small isle, confirmation of the jarls decision to execute her.
The Witcher 2: Assassin of Kings
The Witcher 3: Wild Hunt