Niggun D'veikus L’tefilas Rosh Hashanah is one of the ten niggunim of the Alter Rebbe. It was composed based on a niggun once sung by the Ba’al Shem Tov on Rosh Hashanah.
The Alter Rebbe - and all later Chabad Rabbeim - would sing this soulful niggun during Rosh Hashanah and Yom Kippur davening, especially the first day of Rosh Hashanah before tekiyos.