Who stood in the road with a drawn sword to oppose Balaam?

Answer: The angel of the LORD

The donkey saw the angel of the LORD with a drawn sword; later the LORD opened Balaam's eyes to see him too.

“Then God’s anger was kindled because Balaam was going along, and the angel of the LORD stood in the road to oppose him. Balaam was riding his donkey, and his two servants were with him.”— Numbers 22:22 (BSB)