Merope Gaunt's note

"Instead, he found a clear inch of dust and her note of farewell, explaining what she had done."

- Albus Dumbledore to Harry Potter.

This note was written by Merope Gaunt to her father Marvolo Gaunt and brother Morfin Gaunt, in 1925, after the two men were imprisoned in Azkaban.

Contents
It's likely that one of the contents included Merope saying farewell to her father and brother and explaining that she married the Muggle, Tom Riddle Sr..

Appearances

 * Harry Potter And The Half-Blood Prince