this canticle is a song of penitence, which is placed by its author in the mouth of king Manasseh, who is described in the bible as one of the wickedest kings Judah ever had. the point is not that Manasseh repented (he surely did not), but that this is the sort of prayer he should have prayed, and that God surely hears such prayers of ours.
this canticle is used by the episcopal church in lent, on wednesdays, fridays, and sundays.
it is found in the bible as a separate book in the apocrypha. the version here is that used by the episcopal church, which is a bit shorter than the biblical text.
o Lord and Ruler of the hosts of heaven,
God of Abraham, Isaac, and Jacob,
and of all their righteous offspring:
you made the heavens and the earth,
with all their vast array.
all things quake with fear at your presence;
they tremble because of your power.
but your merciful promise is beyond all measure;
it surpasses all that our minds can fathom,
o Lord, you are full of compassion,
long-suffering, and abounding in mercy.
you hold back your hand;
you do not punish as we deserve.
in your great goodness, Lord,
you have promised forgiveness to sinners,
that they may repent of their sin and be saved.
and now, o Lord, i bend the knee of my heart,
and make my appeal, sure of your gracious goodness.
i have sinned, o Lord, i have sinned,
and i know my wickedness only too well.
therefore i make this prayer to you:
forgive me, Lord, forgive me.
do not let me perish in my sin,
nor condemn me to the depths of the earth.
for you, o Lord, are the God of those who repent,
and in me you will show forth your goodness.
unworthy as i am, you will save me,
in accordance with your great mercy,
and i will praise you without ceasing all the days of my life.
for all the powers of heaven sing your praises,
and yours is the glory to ages of ages. amen.