The Flame Sword was a fiery sword sealed within the Impervious Ice in the Frozen Reaches.
Background[]
Connor melted the ice with the Jewel of Fire, allowing him to take the sword. The nearby Northeast Pillar, when read using the Deciphering Amulet, describes how to remove the sword from the ice: "combine the Jewel of Fire with icy water to produce magical steam to melt the Impervious Ice." It is the only weapon that can kill the Two-Headed Dragon.
Behind the scenes[]
In biblical lore, the fiery sword was the weapon used by Uriel to guard the Garden of Eden.