The Second Ecumenical Council of Lyons of 1274 was one of the largest the Church ever held. Presided over by Pope Gregory X, it was attended by five hundred bishops, observed by kings and ambassadors, and advised by another thousand prelates and periti. On the agenda were the conquest of the Holy Land and the reunion of the Eastern and Western churches, and so it welcomed representatives of the Patriarch of Constantinople, of the Byzantine Emperor and even of the Khan of the Tartars.