The U'Nataneh Tokef is one of the most beautiful and powerful prayers for forgiveness recited during the Days of Atonement between Rosh Hashanah and Yom Kippur. It is said that the prayer was created by a Medieval Rabbi who after being tortured for not converting to Christianity on Yom Kippur dragged himself to his synagogue, recited this prayer and then died. As one would expect of such a prayer, it highlights the fleetingness of human life, particularly when viewed against the Eternal. However it also highlights God's Mercy and willingness to forgive humanity for their sins. (Some might also recognize some of the words from a song by Leonard Cohen).
So now, the Kedushah prayer shall ascend to You, for You, our God, are King.
Let us now relate the power of this day's holiness, for it is awesome and frightening. On it Your Kingship will be exalted; Your throne will be firmed with kindness and You will sit upon it in truth. It is true that You alone are the One Who judges, proves, knows, and bears witness; Who writes and seats, (counts and calculates); Who remembers all that was forgotten. You will open the Book of Chronicles - it will read itself, and everyone's signature is in it. The great shofar will be sounded and a still, thin sound will be heard. Angels will hasten, a trembling and terror will seize them - and they will say, 'Behold, it is the Day of Judgment, to muster the heavenly host for judgment!'- for they cannot be vindicated in Your eyes in judgment.
All mankind will pass before You like members of the flock. Like a shepherd pasturing his flock, making sheep pass under his staff, so shall You cause to pass, count, calculate, and consider the soul of all the living; and You shall apportion the fixed needs of all Your creatures and inscribe their verdict.
On Rosh Hashanah will be inscribed and on Yom Kippur will be sealed how many will pass from the earth and how many will be created; who will live and who will die; who will die at his predestined time and who before his time; who by water and who by fire, who by sword, who by beast, who by famine, who by thirst, who by storm, who by plague, who by strangulation, and who by stoning. Who will rest and who will wander, who will live in harmony and who will be harried, who will enjoy tranquillity and who will suffer, who will be impoverished and who will be enriched, who will be degraded and who will be exalted.
But REPENTANCE, PRAYER and CHARITY remove the Evil of the Decree!
For Your Name signifies Your praise: hard to anger and easy to appease, for You do not wish the death of one deserving death, but that he repent from his way and live. Until the day of his death You await him; if he repents You will accept him immediately.
It is true that You are their Creator and You know their inclination, for they are flesh and blood. A man's origin is from dust and his destiny is back to dust, at risk of his life he earns his bread; he is likened to a broken shard, withering grass, a fading flower, a passing shade, a dissipating cloud, a blowing wind, flying dust, and a fleeting dream.
But You are the King, the Living and Enduring G-d.
There is no set span to Your years and there is no end to the length of Your days. It is impossible to estimate the angelic chariots of Your glory and to elucidate Your Name's inscrutability. Your Name is worthy of You and You are worthy of Your Name, and You have included Your Name in our name.
(I found the English translation of this prayer here: http://www.ou.org/chagim/roshhashannah/unetanehtext.htm)