The wishsong came into being when Wil Ohmsford used the Elfstones to protect the Chosen Amberle Elessedil during her quest to find the Bloodfire and restore the Ellcrys. Because Wil was only one-eighth Elf, he did not have sufficient Elven blood to use the Elfstones easily and every time he used the Stones their magic entered his blood, changing him in a way he could not understand.
Wil finally realized the nature of the change the Elfstones had inflicted on him when his children, Brin and Jair Ohmsford, were born. Both had inherited the Elven magic in their father's blood in the form of the wishsong.
Named by Brin, the wishsong is a magic that uses the voice to make one's desires come true. Brin could alter reality simply by wishing for something and singing for it to happen. As a child she sang to change the leaves of an old maple tree from green to brown in the summer. During her quest to destroy the Ildatch, she was able to use the wishsong to heal Rone Leah of a deadly poison, control the moor cat Whisper, gain entry into the Maelmord, and destroy the Ildatch.
While Brin could alter the world, Jair could only create illusions with the wishsong. However, he was able to use his powers to save Brin from the Ildatch's influence, and in later years he learned to inhabit his illusions and take on the qualities and powers of the illusions he created.
Centuries later, the wishsong resurfaced in Par Ohmsford, a descendant of Jair. At first he could only use the wishsong like Jair did, to create images, but it soon evolved into an extremely powerful weapon, becoming more like the magic of Brin. Par used the wishsong to seek out the Sword of Shannara and later to destroy Rimmer Dall and the Shadowen.
Several generations later, the wishsong reappeared in the siblings Bek and Grianne Ohmsford. Grianne used the wishsong for malevolent purposes during her time as the Ilse Witch but reformed herself when she became Ard Rhys of the third Druid Order. Bek's son Pen Ohmsford grows up without the magic of the wishsong, but his use of the darkwand awakens the wishsong in him. He uses the power to help save his aunt Grianne from the Forbidding.
Almost a century later, Pen's grandsons Railing and Redden Ohmsford used the wishsong during Khyber Elessedil's quest for the missing sets of Elfstones. Redden used his wishsong to unlock the power of the crimson Elfstones, enabling him to use their magic in combination with the wishsong.
The wishsong would reappear several generations later in Redden's descendant Reyn Frosch, but in a more volatile and violent form. While not explicitly stated, it is clear that Redden's use of the Elfstones infused his blood with their magic, similar to what had happened with Wil Ohmsford centuries earlier, and that this might explain the change in the nature of the wishsong in Reyn. Railing's descendant Chrysallin Leah also possesses the wishsong.
While the wishsong is an extremely potent and versatile magic, like all Elven magic it depends on the strength of the user and is therefore not limitless. It can also be dangerous, as the power of the magic can take over the user and cause them to lose their mind and give in to the magic's dark influence. Brin faced this problem during her quest to destroy the Ildatch, and Par nearly gave in to Rimmer Dall when he lost control of the magic. Grianne was ultimately subverted completely, becoming the Queen of the Forbidding and the Demons within it. Redden, Chrysallin, and Reyn all fell into varying levels of catatonia after losing their control over the wishsong's power.