Who set Peter free from Herod's prison the night before his trial?

Answer: An angel of the Lord

An angel woke Peter, his chains fell off, and the iron gate to the city opened for them by itself.

“Suddenly an angel of the Lord appeared and a light shone in the cell. He tapped Peter on the side and woke him up, saying, “Get up quickly.” And the chains fell off his wrists.”— Acts 12:7 (BSB)