Eighth Ecumenical Council
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 was 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"), as well as Fr. John Meyendorff, Fr. George Dragas and Metropolitan Hierotheos (Vlachos) of Nafpaktos.
One of the first references as "Eighth Ecumenical Council" is to be made in 15th century by St. Mark of Ephesus, who expresses the general theological view of that time in Constantinople during the so-called 'robber-council' in Ferrara-Florence (to be referenced in Pedalion comments for the 879-880 "Synod gathered in Agia Sophia").
Further, the 1848's 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."
The Rule of Faith
This council occurred during the time period where the filioque addition was still gaining acceptance in the Latin church, and seems to have reiterated the original creed with language specifically targeted at denouncing the filioque addition. This is particularly interesting in light of the full papal endorsement of the council for many centuries, before it was eventually repudiated.
Following is the rule of faith which appears in the minutes of the 6th and 7th acts of the council:
"Jointly sanctifying and preserving intact the venerable and divine teaching of our Lord and Savior Jesus Christ, which has been established in the bosom of our mind, with unhesitating resolve and purity of faith, as well as the sacred ordinances and canonical stipulations of his holy disciples and Apostles with an unwavering judgement, and indeed, those Seven holy and ecumenical Synods which were directed by the inspiration of the one and the same Holy Spirit and effected the [Christian] preaching, and jointly guarding with a most honest and unshakable resolve the canonical institutions invulnerable and unfalsified, we expel those who removed themselves from the Church, and embrace and regard worthy of receiving those of the same faith or teachers of orthodoxy to whom honor and sacred respect is due as they themselves ordered. Thus, having in mind and declaring all these things, we embrace with mind and tongue and declare to all people with a loud voice the Horos (Rule) of the most pure faith of the Christians which has come down to us from above through the Fathers, subtracting nothing, adding nothing, falsifying nothing; for subtraction and addition, when no heresy is stirred up by the ingenious fabrications of the evil one, introduces disapprobation of those who are exempt from blame and inexcusable assault on the Fathers. As for the act of changing with falsified words the Horoi (Rules, Boundaries) of the Fathers is much worse that the previous one. Therefore, this holy and ecumenical Synod embracing whole-heartedly and declaring with divine desire and straightness of mind, and establishing and erecting on it the firm edifice of salvation, thus we think and loudly proclaim this message to all:
[the original Nicene-Constantinopolitan Creed is cited here, without the filioque addition]
Thus we think, in this confession of faith we were we baptized, through this one the word of truth proved that every heresy is broken to pieces and canceled out. We enroll as brothers and fathers and coheirs of the heavenly city those who think thus. If anyone, however, dares to rewrite and call Rule of Faith some other exposition besides that of the sacred Symbol which has been spread abroad from above by our blessed and holy Fathers even as far as ourselves, and to snatch the authority of the confession of those divine men and impose on it his own invented phrases and put this forth as a common lesson to the faithful or to those who return from some kind of heresy, and display the audacity to falsify completely the antiquity of this sacred and venerable Horos (Rule) with illegitimate words, or additions, or subtractions, such a person should, according to the vote of the holy and Ecumenical Synods, which has been already acclaimed before us, be subjected to complete defrocking if he happens to be one of the clergymen, or be sent away with an anathema if he happens to be one of the lay people." 
- John Meyendorff, "Rome and Orthodoxy: Is 'Authority Still the Issue?," Living Tradition, St. Vladimir's Seminary Press, 1978, pp. 63-80.
- The Eighth Ecumenical Council: Constantinople IV (879/880) and the Condemnation of the Filioque Addition and Doctrine by Protopresbyter George Dion Dragas
- The Eighth and Ninth Ecumenical Councils
- The Eighth Ecumenical Council:Constantinople IV (879/880) and the Condemnation of the Filioque Addition and Doctrine