Malla's Soul Stone is a sort of ioun stone which can be equipped in the head slot. It raises the Dexterity and Constitution of the wearer by one point and can be used once per day to cast the spell Neutralize Poison. This item appears in Baldur's Gate II: Throne of Bhaal and Baldur's Gate II: Enhanced Edition.
It can be gained during the side quest Rescue Marlowe's Daughter, whose soul is in the possession of the lich in Amkethran. Because her soul powers the sapphire, characters whose Alignments are good can't wear it.
To obtain this item during the quest, Marlowe has to fulfill his part of the bargain struck with the lich. After this is done, the lich will give it you and then you are free to decide whether to keep the stone or to complete the quest.
|“||The small star sapphire gem that holds the soul of the innocent young woman, Malla, has the magical ability to float about the forehead and act as an ioun stone of considerable power. Only those of little conscience would consider using this device, however.||”|