From Rolling Stone:
“The legend of Rhiannon is about the song of the birds that take away pain,” Nicks said in 1980. “That’s what music is to me. I don’t want any pain.” After reading about the Welsh witch Rhiannon, Nicks wrote this song for her, saying, “If I didn’t know she was a mythological character, I would think she lived down the street.”
Nicks discovered Rhiannon through a novel called Triad, by Mary Leader. The novel is about a woman named Branwen, who is possessed by another woman named Rhiannon. There is mention of the Welsh legend of Rhiannon in the novel, but the characters in the novel bear little resemblance to their original Welsh namesakes (both Rhiannon and Branwen are major female characters in the medieval Welsh prose tales of the Mabinogi).
Nicks bought the novel in an airport just before a long flight and thought the name was so pretty that she wanted to write something about a girl named Rhiannon. She wrote “Rhiannon” in 1974, three months before joining Fleetwood Mac, and has claimed it took 10 minutes to write.
After writing the song, Nicks learned that Rhiannon was a euhemerised Welsh goddess, and was amazed that the haunting song lyrics applied to the Welsh Rhiannon as well. Nicks researched the Mabinogion story and began work on a Rhiannon project, unsure of whether it would become a movie, a musical, a cartoon, or a ballet. There are several “Rhiannon Songs” from this unfinished project including “Stay Away” and “Maker of Birds”. Nicks wrote the Fleetwood Mac song “Angel” based on the Rhiannon story.
Nicks avoided wearing black clothing for “about two years” in an effort to distance herself from the witchcraft and dark arts associations surrounding her as a result of the lyrics to “Rhiannon” giving fans the wrong impression.
From allmusic, reviewing the album:
It’s unfair to say that Fleetwood Mac had no pop pretensions prior to the addition of Lindsey Buckingham and Stevie Nicks to the lineup in 1975. When they were lead by Bob Welch they often flirted with pop, even recording the first version of the unabashedly smooth and sappy “Sentimental Lady,” which would later be one of the defining soft rock hits of the late ’70s. Still, there’s no denying that 1975’s Fleetwood Mac represents not just the rebirth of the band, but in effect a second debut for the group — the introduction of a band that would dominate the sound of American and British mainstream pop for the next seven years. In fact, in retrospect, it’s rather stunning how thoroughly Buckingham and Nicks, who had previously recorded as a duo and were romantically entangled in the past, overtook the British blues band. As soon as the Californian duo came onboard, Fleetwood Mac turned into a West Coast pop/rock band, transforming the very identity of the band and pushing the band’s other songwriter, keyboardist Christine McVie, to a kindred soft rock sound. …Fleetwood Mac wrote the blueprint for Californian soft rock of the late ’70s and was the standard the rest were judged by.