Each Rosh Hashana and Yom Kippur, we daven the Unesaneh Tokef as part of the Mussaf tefillah.
The story behind the Unesaneh Tokef tefillah (expressing the seriousness of the day) relates to the Gezeira Rah (evil decree) of forced conversion to Christianity which was faced by R’ Amnon of Mainz and his Kehilla roughly 1,000 years ago.
R’ Amnon feared for his Kehilla due to having faced persistent dread coercion as well as potential mortal, imminent threat of communal harm or deaths hanging over them as retribution for failure to convert to Christianity. Therefore, at one point, he weakened by giving the Bishop or Governor (whatever the correct title was) the perception of his consideration of possible compliance with the demand to convert. Almost immediately after taking leave of the Bishop, R’ Amnon became distraught feeling that he betrayed Hashem by having given the impression that he would consider conversion and give an answer to the Bishop 3 days hence.