What is the importance of the Sword of Leah in the novel, The Wishsong of Shannara?

Rone Leah carries the Sword of Leah. It had been used by Rone's great-grandfather, and Rone seeks to live up to his image. Allanon adds magic to the sword, instructing Rone to dip it in the waters of the Hadeshorn. The sword then contains magic to deflect attacks from Mord Wraiths and other dark magic.