Always and Forever is the vow that was made long ago by members of the Mikaelson Family, and members of Dahlia and Esther's Family Bloodline, specifically a promise made between Esther Mikaelson and her sister Dahlia in the late 10th century, during which they vowed to never leave each other's side. "Always and Forever" was then used as a promise again by Esther to her son Klaus many years later to assure him that she would always be there for him if he was scared.
Later Klaus, Elijah and Rebekah made the same promise to stick together, after their family had split up following the death of their mother. This promise, in any iteration, has been broken sixteen known times in its history - 2 times by Esther in the late 10th century, 5 known times by Rebekah in 1919, in 2011, in 2012 and in 2013, 2 known times by Elijah in 2012 and in 2013, and 7 known times by Klaus in 1001, in 1835, in 1922, in 2010 (twice), in 2011 and in 2013.
In the 21st century, the original promise was renewed between Elijah, Klaus, Rebekah, and their brother Kol after they were reunited as siblings for the first time in years. Klaus reminded his maternal younger half-brother of this promise again in I Love You, Goodbye, when it became clear that Kol was dying from a hex. In Behind the Black Horizon, it is cemented that this vow also includes Freya, Finn, and Alex.
The vow was then again renewed by Marcel Gerard, Hope Mikaelson, Jaxon Salvatore, Nikolina Gilbert-Salvatore, and Lillian Gilbert-Salvatore in 2019