From the lore of Morrighan, First Daughter is an honorific accorded to royal eldest daughters born with the gift, or "Siarrahs". The gift is described as a "way of knowing," and resembles that of prophecy or psychic foreknowledge. It can manifest as visions, sounds, or feelings.
While the phrase itself can naturally apply to any first born female, it has a mystical and reverent assignation in Morrighan, particularly with respect to the first born daughters of the royal line. Descended from the historical figure Morrighan, the woman who led The Chosen Remnant to the land that would become the kingdom of Morrighan, the Royal Family has a special place in the religious life of the kingdom, and first born daughters of the line are accorded deep respect with or without the gift.
Lia is the current First Daughter of the House of Morrighan.