This is the original Latin hymn, based on the Great O Antiphons, which gave rise to the many versions of O Come O Come Emmanuel. I am uncertain of its date of composition - however, the form suggests that this is very late Latin. The original Latin chanted in cathedrals and abbeys would have been that of the Antiphons themselves.

Veni, veni Emmanuel!
Captivum solve Israel!
Qui gemit in exilio,
Privatus Dei Filio.
Gaude, gaude, Emmanuel
Nascetur pro te, Israel.


Veni, O Jesse Virgula;
Ex hostis tuos ungula,
De specu tuos tartari
Deduc et antro barathri

Veni, veni O Oriens!
Solare nos adveniens,
Noctis depelle nebulas,
Dirasque noctis tenebras.

Veni clavis Davidica;
Regna reclude caelica;
Fac iter tutum superum,
Et claude vias inferum.

Veni, veni Adonaï!
Qui populo in Sinai
Legem dedisti vertice,
In Maiestate gloriae.

Everything Hymnal