In 1 Chronicles 21, what did David see standing between heaven and earth with a drawn sword over Jerusalem?

Answer: The angel of the LORD

Seeing the angel with his sword stretched out over Jerusalem, David and the elders, clothed in sackcloth, fell facedown.

“When David lifted up his eyes and saw the angel of the LORD standing between heaven and earth, with a drawn sword in his hand stretched out over Jerusalem, David and the elders, clothed in sackcloth, fell facedown.”— 1 Chronicles 21:16 (BSB)