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 (often the main characters in the series). The Wishsong is a magical side effect of Wil Ohmsford's prolonged use of the Elfstones with predominantly human blood. These "side effects" resulted in Wil's children Brin Ohmsford and Jair Ohmsford being born with two different strains of innate magical power later named the "Wishsong".
The Wishsong is controlled through both desire and sound. The user must first desire the intended effect. The Wishsong's magic then manifests itself through the vocal cords of the user. Users typically are unaware of what sounds are necessary to generate a particular effect, though the user can use subdued or intensified vocals (whispers or shouts) to create subtle uses or intense bursts of magic. Gags or other devices which prevent full vocal control also prevent use of the Wishsong.
Wil's children each received a strain of the Wishsong which developed independently through the series. The two strains of the Wishsong passed through the lineal descendants of the respective siblings Brin and Jair. Generational gaps did occur in each line, the Wishsong "choosing" who would be the next recipient.
The first strain was that of Brin's Wishsong which, when used, allowed Brin to alter the appearances and behavior in living organisms. Uses included rapidly aging trees, communicating with animals, induced pain, death, curing disease and injury, and in one instance shattering trees by forcing the trees to rapidly reshape. An important note is that Brin's strain only affected living organisms.
At the close of Wishsong of Shannara Allanon requests, and Brin promises, to discontinue use of the Wishsong. Brin's Wishsong gradually weakened in power through non-use by the lineal holders. The final holder of Brin Ohmsord's strain of the Wishsong was limited to communication with animals, though it is alluded that the Wishsong granted Walker greater affinity in learning druidic and alchemy magics. Brin's legacy died with Walker Boh.
Jair's use of the Wishsong was limited to the changing of appearances - ie illusions. Jair was unable to generate any actual change in matter without aid. On a single occasion, through the use of a magical trinket, Jair was able to transport a semi-corporal image (illusion) of himself to appear to Brin in order to break the Ildatch's control over Brin.
Unlike Brin's strain of the Wishsong, Jair's heirs did not refrain from continued use of the Wishsong. Seemingly, the Wishsong's use remained limited to manifesting illusion. Jair's legacy had, in fact, been evolved through the generations. As of Par Ohmsford's receipt of the Wishsong, the magic and had "evolved" full circle. During a battle in Scions of Shannara Par used the Wishsong to generate magical blue energy blasts similar to the offensive capabilities of the original blue Elfstones (the seeking capabilities of the blue Elfstones did not manifest). Quickly after discovering the new use, Par developed severe mental and emotional strain throughout the later three books of the Heritage series.
Jair's legacy continued to evolve through Grianne Ohmsford, Bek Ohmsford, and Penderrin Ohmsford. During the Voyage series, the Wishsong had no discernible limit other than the innate character weaknesses of the wielder. Uses included altered behavior, force blasts, generate illusions, induce pain, and morphing living tissue to create beasts. During one section of his journey in the Voyage series, Bek was able to navigate through a canyon with help from the Sword of Shannara. It is unclear as to whether the Sword was providing navigational awareness or if the Sword was allowing for greater use of the Wishsong (or a combination of the two).
Of note, the Voyage saw significant changes without explanation to previously set rules regarding the rules, use, and limits of magic and magical items provided in original and Heritage prior series.