What did God promise David in the Davidic covenant?
God promised David a covenant that through him would come a royal family bloodline, and through that royal family would come the King of Israel. This covenant established David's line as the royal lineage, later traced through Solomon and Nathan. It assured that Israel's coming King would arise from David's family.
Les teaches that about 1,000 BC, roughly 500 years after the law was given, God made another covenant with David. This Davidic covenant promised that through David would come a royal family bloodline, and through that royal family would come the King of Israel. Les notes that when you follow the genealogies coming out of David, you find Solomon and Nathan, with David and Bathsheba having two sons, Solomon on one side and Nathan on the other. The point is that God was establishing the royal lineage through which Israel's King would come.