Dallah Thaun, the Lady of Mysteries, is Yondalla's dark, hidden aspect. She is the great secret of the halfling race, who do not share knowledge of Dallah Thaun with outsiders. She was physically split from Yondalla when she created (or discovered) the halfling race, but she and Yondalla are still one being. What one knows, the other knows, and those that worship Dallah Thaun are also worshipping Yondalla (and vice versa).