The fourth of the O Antiphons.

O Clavis David,
et sceptrum domus Israel,
qui aperis, et nemo claudit,
claudis, et nemo aperuit:
veni, et educ vinctum
de domo carceris,
sedentem in tenebris,
et umbra mortis.

In English:

O Key of David,
And scepter of the house of Israel,
You open, and no one closes,
You close, and no one opens:
Come, and lead the prisoner
From jail,
Who sits in darkness
And the shadow of death.