The Eighth Ecumenical Council was a reunion council held at Constantinople in 879-880. This council was originally accepted and fully endorsed by the papacy in Rome (whose legates were present at the behest of Pope John VIII), but later repudiated by Rome in the 11th century, retroactively regarding the robber council of 869-870 to be ecumenical. The council of 879-880 affirmed the restoration of St. Photius the Great to his see and anathematized any who altered the Nicene-Constantinopolitan Creed, thus condemning the Filioque.
This council is not regarded as ecumenical by all Orthodox Christians, but some major voices in the Orthodox world do so, including 20th century theologians Fr. John S. Romanides and Fr. George Metallinos (both of whom refer repeatedly to the "Eighth and Ninth Ecumenical Councils"), Fr. George Dragas and Metropolitan Hierotheos (Vlachos) of Nafpaktos.
Further, the Encyclical of the Eastern Patriarchs refers explicitly to the "Eighth Ecumenical Council" regarding the synod of 879-880 and was signed by the patriarchs of Constantinople, Jerusalem, Antioch, and Alexandria as well as the Holy Synods of the first three.
Those who regard these councils as ecumenical often characterize the limitation of Ecumenical Councils to only seven to be the result of Jesuit influence in Russia, part of the so-called "Western Captivity of Orthodoxy."
An interesting external attestation to the consideration of this synod to be the Eighth Ecumenical Council is the Roman Catholic Church's Catholic Encyclopedia (1907), which describes the council of 879-880 as the "Pseudosynodus Photiana," noting that the "Orthodox count [it] as the Eighth General Council."