'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 because there is not a God in Israel, that you go to inquire of Baal-zebub 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 struck in the camp of the Assyrians a hundred fourscore and five thousand: and when they arose early in the morning, behold, these were all dead bodies.