A letter wrote by the alternate version of Thomas Wayne, the Batman from the Flashpoint timeline it was the only object than survived the erasing of that timeline.
The Letter was entrusted to Barry Allen by Thomas Wayne to give to his son Bruce Wayne during the last battle of the Flashpoint earth. This would have been the last thing Bruce would have from his Father.
The letter is shown in Flashpoint #1, it's the narrator captions. In order it would read like this.
There's only one thing I know about life.
I know some things happen by chance,
And some things happen because we make them happen.
Barry Allen was a man haunted by his past.
But when he became the Flash, he left the ghosts behind.
He found love. A Family. Friends.
Barry thought yesterday was behind him...
...But somebody wouldn't let him escape it.
When Barry came to me for help, I turned him away.
I'm not the hero of this story.
I'm a man who's been corrupted by his own unbearable pain,
I'm a man who has too much blood on his hands to be called good.
I'm a man who had nothing to live for...
...until the day I met the Flash.
The first time I met Barry Allen, I nearly killed him.
Like I said, I'm not the hero of this story.
After receiving and reading the letter Bruce Wayne breaks into tears over the letter and thanks Barry for being a good friend and for delivering this letter to him.
The letter eventually it was destroyed by the Reverse Flash, Eobard Thawne, during the Button arc.