What are the Lann Dàn, and how do they function in the series?

Pronounced Lanna Dawn, the Blades of Destiny are magical Tuatha long blades that were crafted by Badb Catha, the Tuatha Dark Goddess, to aid the Sun God, Lugh, in his battle with the Fomorians. The Blade portion of each weapon is a diamond pulled from deep within the earth on the Isle of Fire and Ice. In Blades and Stone of Destiny, they cast balefire that can overcome magical Fomorian defenses. Badb uses the Lann Dàn to act as a conduit between the Stone of Destiny and the Heart of Destiny as one of the Triple Dàns. She later re-envisions them to function in the mortal world to cast balefire at non-magical targets, such as people and objects (with devastating effectiveness).