The angel continues speaking to Daniel.
A commander of the king of the South will become the king of the North.
The king of the South will make an alliance with the king of the North. This alliance would be a formal agreement that both nations are required to follow. AT: "the king of the South and the king of the North will promise to work together"
The king of the South will give his daughter in marriage to the king of the North. The marriage will confirm the agreement between the two kings.
Here "arm" stands for power. (See: rc://en/ta/man/translate/figs-metaphor)
This appears to refer to a plot to kill her and those who made the alliance. This phrase may be expressed in active form. AT: "They will abandon her" (See: rc://en/ta/man/translate/figs-activepassive)