666 was created with lyricist Costas Ferris who was not part of the band. The music combines psychedelic and progressive rock with ethnic instruments, choral chanting, recitations, and use of electronic keyboards. The record company, Mercury Records, objected to the double-album length and to the musical experimentation, as well as the track "∞" (infinity), on which a female orgasm is simulated by the Greek actress Irene Papas, who repeated the words "I was, I am, I am to come" over a sparse percussion track.
After Roussos and Sideras had already released their first solo albums, Mercury agreed to release 666, one year after completion and two years after the band had finished their parts in it, on its progressive rock subsidiary Vertigo Records. The single "Break" also came out in 1972, as did a single vinyl edition of the album in Brazil, also titled Break. An extended vinyl edition of 666 was released in Greece in 1974, containing alternative mixes of songs with music cut from other versions of the album. Some of these versions had appeared on the Brazilian release.
AllMusic gives it 4½ stars, but notes that "the entire set eventually becomes too overwhelming to sit through". IGN rated the album #3 on their list of Top 25 Prog Rock Albums. In the Q & Mojo Classic Special Edition ‘’Pink Floyd & The Story of Prog Rock’’, the album came #40 in its list of "40 Cosmic Rock Albums".