The Wishsong is a form of magic that exists in Terry Brook's fictional world of Shannara. The Wishsong is wielded by many members of the Ohmsford family, including Brin, Jair, Par, Grianne and Bek. The Wishsong came into being when Wil Ohmsford used the Elfstones to kill a demon to protect Eretria and Amberle Elessedil, but he did not have sufficient Elven Blood to use them properly and the result was that his children Brin Ohmsford and Jair Ohmsford and many of their descendants were born with the Wishsong.
The Wishsong is a magic that is controlled by singing. Brin Ohmsford could use it to dramatically change nature, in her childhood, she changed the leaves of a tree from Green to Brown in the summer, or it could be used as a weapon. Jair Ohmsford could only use it to create illusions. Par Ohmsford at first could only use the wishsong to create images but it evolved into an extremely powerful weapon, which he ultimately used to destroy Rimmer Dall and all of the Shadowen. Other Ohmsford family members have the power of the Wishsong as well, either in the form to create illusions or as a powerful weapon.
One such good example of how the wishsong works appears in Ilse Witch. Truls Rohk tells Bek Ohmsford to hum and will him backwards, the result was that Truls Rohk moved backwards slightly, this is how Bek Ohmsford came to learn he had the magic.
The Wishsong is extremely potent and can be dangerous, it took over Par Ohmsford in The Talismans of Shannara and he nearly gave in to Rimmer Dall and let him come into Par's mind to help him control it.