Since I am not a native speaker, I would like to know whether you use such an expression 'as the dot on the i'? If not, which expression could be similar?
For example, after a series of unfortunate events, the last unfortunate event was 'as the dot on the i'.
I know that there is 'dot the i's and cross the t's', but that has a different meaning.
To express something negative in the sense of "another indignity" there is the final insult.
Although I couldn't find a good reference to a dictionary definition of the term, it's something I commonly hear.
There are many examples of its use. All bold text in the following is my own emphasis.
"The Final Insult: A Lost Domain" by Paul Boutin:
They lost their jobs, their careers, even their apartments in the Internet bust. But the final insult for many Web workers is learning that somewhere in the churn of e-mail addresses and phone numbers from the dot-com bust, they've lost their personal domains as well.
Pushing the Pencil by Donald Wilcox Thomas:
But now comes the final insult: teachers are to be paid according to their "performance," meaning what they are able to produce in the minds of their students. Now they are to be held suddenly "accountable," meaning that their teaching is to be translated into numbers that are duly extracted from certain tests designed to weigh and measure the quality of human thought, presuming that such tests actually exist. It will be like piece work on earns in a factory based on the number of widgets produced.
Dark Angel by Lynne Graham:
'I made all the allowances, I did all the giving, and at the end of the day you still let me down. You let a woolly-headed old man play ducks and drakes with my money … the final insult for me has to be the sight of that bloody tarpaulin on the roof!'
"What counts as a genocide" by H.J. at The Economist:
The “g-word” has considerable power. If mass slaughter is recognised as genocide when it is happening, it is harder for outside forces to sit idly by. When it is over, official recognition that it was genocide can give the survivors some grim satisfaction. But when that recognition is withheld, whether because of a technicality or political expediency, it can feel like the final insult. And some human-rights activists and legal scholars feel that genocide’s status as the “crime of crimes” sometimes overshadows the horror of other crimes against humanity.
In a slightly different sense—one that speaks more to a reaction of cynical defeat (or possibly of anger) than of simply the event itself, there is the last straw, as suggested in a comment under the question.
[Merriam-Webster]
: the last of a series (as of events or indignities) that brings one beyond the point of endurance
// But at 84, widowed and slowing down, the last straw for my mother was an errant bat in the bedroom at midnight.
— Lee Woodruff, San Diego Union-Tribune, "A summer vacation caregiver," 8 Aug. 2019
