'Angel of the Lord' in the Bible

But the angel of the LORD said to Elijah the Tishbite, Arise, go up to meet the messengers of the king of Samaria, and say unto them, Is it not because there is not a God in Israel, that ye go to inquire of Baalzebub the god of Ekron?

And the angel of the LORD said unto Elijah, Go down with him: be not afraid of him. And he arose, and went down with him unto the king.

And it came to pass that night, that the angel of the LORD went out, and smote in the camp of the Assyrians an hundred fourscore and five thousand: and when they arose early in the morning, behold, they were all dead corpses.