On the HBO series, Lightbringer is a sword Stannis Baratheon acquires when Melisandre encourages him to pull the blade out of the fire created by the burning statues of the Seven. According to her prophecy, Lightbringer could only be pulled from fire by the Lord of Light's chosen one. After Stannis dies at the hands of Ramsay's army, the fate of the weapon is unclear.
In the novels, the sword has yet to exist as anything more than a legend. Forged by Azor Ahai, a Messiah figure in the Lord of Light faith, the sword glows and radiates a heat that harms all those deemed unworthy.