Introit for the Feast of St. Nicholas, one of five antiphons used in the Latin Eucharist. Statuit ei Dominus is an introit for a martyr, pope or bishop. It is in Old Roman Chant, a predecessor to Gregorian chant.
Statuit ei Dominus testamentum pacis,
et principem fecit eum;
ut sit illi sacerdotii dignitas in aeternum.
The Lord made unto him a covenant of peace,
and made him a prince,
that the dignity of the priesthood should be to him forever.
From Callixtinus, YouTube.