The Red Sword is one of several names given to a comet of unusual brightness that appeared in the night skies over Westeros and Essos in the year 299 AL, during the early part of the War of the Five Kings. It was also called the Dragon's Tail, Prince Joffrey's Comet (in King's Landing), Mormont's Torch (amongst the sworn brothers of the Night's Watch) and, amongst the Dothraki, the Bleeding Star, shierak qiya in their tongue.
The comet was a vivid red in colour with an unusually bright tail pointing to the south-east. Whilst it was visible across most of the known world and its presence was interpreted in many different ways, its existence seemed to be the most portentous for Daenerys Targaryen and her small army of followers. The comet was first noticeable on the night that her dragons hatched, and she decided to follow the tail of the comet looking for salvation, which eventually led her party across the Red Wastes to Vaes Tolorro and afterwards to the great city of Qarth on the Jade Sea. After this time the comet lost intensity and eventually faded away around the time Daenerys took ship for Slaver's Bay.
Despite its impressive size and vividness, the comet was explained as a purely scientific phenomenon by the maesters of the Citadel, whilst the various religious orders had their own explanations for its significance.