Isildur's Bane was a name for the One Ring. After Sauron's defeat in the War of the Last Alliance, Isildur, Elendil's son took the Ring, and later journeyed northward with it to his kingdom of Arnor. On 5 October T.A. 2 his small party was ambushed by Orcs. Isildur's small force was cut down and he fled the battle wearing the Ring. It slipped from his finger, though, and so both Isildur and the Ring were lost.
On 20 June 3018, Sauron attacked Osgiliath. On the eve of this assault a dream came to Faramir, Boromir's brother, that warned of the awakening of Isildur's Bane. Faramir had the dream often and Boromir had it once. After consulting with Denethor, Boromir journeyed for one hundred and ten days to seek advice from Elrond. Finally on 25 October Boromir received the answers to his questions at the Council of Elrond, where Frodo Baggins brought out the Ring and Elrond proclaimed, "Behold Isildur's Bane!"