The Harp of King Fflewddur Fflam was a magic harp given to Fflewddur Fflam after his failed trial to become an official bard. The harp's strings would break whenever Fflewddur told a lie or "colored the facts" (a frequent occurrence); as partial compensation, the instrument also possessed a beautiful tone and often seemed to play of its own will when a tune was begun. After Fflewddur's heroism against the Horned King, Gwydion gifted him with a single harp string that would never break, no matter how many extravagances he laid upon it.
During the events of The High King, Fflewddur broke the harp for firewood to save his friends' lives. The only piece to survive was the string given by Gwydion; it was given to Taran as a farewell gift.
The story of how Fflewddur came to acquire the harp is told in the short story The Truthful Harp.