This is the sixth of the Great Antiphons/O Antiphons sung at the Magnificat at Vespers during the last days of Advent. This antiphon is used on December 22. (In the Paris Breviary and in medieval English usage it is on December 21.)
O Rex Gentium, et desideratus earum, lapisque angularis, qui facis utraque unum: veni, et salva hominem, quem de limo formasti.
Words: Anonymous, ca. 9th C.
Tune: “O Rex Gentium” Gregorian Chant, Mode II, traditional.
This is the English translation of the text as it appears in Fr. Matthew Britt’s 1922 text of The Hymns of the Breviary and Missal:
O King of the Gentiles, yea, and the desire thereof, the Corner-stone that makest both one: come and save man, whom Thou hast made out of the slime of the earth.