Sources for Un'taneh Tokef

(5) And a throne shall be established in goodness

(7) They are dreadful, awesome; They make their own laws and rules.

(16) God of Yisrael,

(17) Shaddai, Dread and Awesome,

On the first day of Tishrei, New Year's Day, sentence is pronounced upon the countries of the world - those destined for war and those destined for peace, those for famine and those for plenty, those for death and those for life; on this day the lives of mortals are scrutinized to determine who is to live and who is to die.This day was chosen because the first human was created on Rosh ha-Shanah.In the first hour it occurred to Gd to create him...in the ninth, Gd gave him a command; in the tenth, he transgressed..in the eleventh, he was brought to judgement;in the twelfth, Gd pardoned him.The Holy One said to him:Let this be an omen for your descendants that as you entered this day for judgement and were pardoned so will your descendants come before Me in judgment on this day and be pardoned.When will this be?"In the seventh month, on the first day of the month."(Lev 23:24)

(12) "without wrong": When a man departs from the world, all his deeds come and present themselves to him one by one, saying: "This is what you did on this and this day. Do you believe it?" The man: "Yes, I do." The deed: "Sign!" — and he signs, viz. (Iyyov 37:7): "He makes the hand of every man sign."

For #5 For great is the day of the Lrd, and most awesome (Joel 2.11). This is Yom Kippur. And who can endure it? For R' Krispa said in the name of R' Yohanan, There are 3 account books. One is for completely righteous, one for completely evil. And one is for those in the middle. These are for eternal life (Daniel 12.2). Rebbe said, These are the completely righteous. And these for the disgrace of eternal punishment(ibid). These completely evil 'will be erased from the book'(Ps 69.29). These are the evil. Alive(ibid). These are the righteous. And with the righteous they will not be inscribed(ibid). These are the those in between, to whom the Holy Blessed One gave 10 days from Rosh HaShanah to Yom Kippur. If they do t'shuvah, they are written with the righteous. If not, with the evil.

Rabbi Yitzḥak said: A person’s sentence is torn up on account of four types of actions. These are: Giving charity, crying out in prayer, a change of one’s name, and a change of one’s deeds for the better.

And some say: Also, a change of one’s place of residence cancels an evil judgment

65b Three things abrogate the harsh decree: t'fillah and tz'dakah and t'shuvah...

(13) As a father has compassion for his children, so the LORD has compassion for those who fear Him. (14) For He knows how we are formed; He is mindful that we are dust. (15) Man, his days are like those of grass; he blooms like a flower of the field; (16) a wind passes by and it is no more...(17) But the LORD’s steadfast love is for all eternity ... (19) The LORD has established His throne in heaven, and His sovereign rule is over all.