Why is Isildur's Bane such a big deal?

Kalimac,

After it was reforged at Rivendell, it was renamed Anduril in its second incarnation...

I would like to find out more about the time when Narsil became Anduril. I am re-reading FotR and don't remember that event happening during The Council of Elrond. Maybe it was mentioned in one of the other books? Would you happen to have a reference for me or could someone else help out. Many thanks [img]smilies/smile.gif[/img]