Barrage is an Expert Marksman / Huntsman skill in Divinity: Original Sin and Divinity: Original Sin II respectively.
Original Sin
Fire multiple arrows in a straight line, each dealing 60% (75% in the Enhanced Edition) of the character's damage.
Locations
Only one book exists.
Original Sin 2
Fire three arrows at three targets of your choice, each dealing physical damage.
Scales off of basic attack damage and Strength.