Interestingly, the Tuatha de Danaan did not, according to all the legends, have to lose this battle--they chose to. They were in possession of four invincible "weapons:" the Sword of Nuada (almost certainly the ancestor of King Arthur's Excalibur), which never failed to strike its target and never failed to deliver a fatal wound; the Spear of Lugh (the ancestor of the Spear of Longinus), which, if commanded to kill by its owner, would fly among the enemy slaying them until recalled; and the Cauldron of the Dagda, which could never be emptied of food--a true boon to any army. The fourth item, much more enigmatic, was the Stone of Fal, which was reputed to "cry out under a rightful king."

In any case, they never used them, any of them, in this battle.