20 Now Herod was angry with the people of Tyre and Sidon, and they came to him with one accord, and having persuaded Blastus, the king's chamberlain, they asked for peace, because their country depended on the king's country for food. 21 On an appointed day Herod put on his royal robes, took his seat upon the throne, and delivered an oration to them. 22 And the people were shouting, “The voice of a god, and not of a man!” 23 Immediately an angel of the Lord struck him down, because he did not give God the glory, and he was eaten by worms and breathed his last.
24 But the word of God increased and multiplied.
25 And Barnabas and Saul returned from  Jerusalem when they had completed their service, bringing with them John, whose other name was Mark. (Acts 12:20-25 (ESV)
More can be learned of Herod’s death in the works of Josephus. Herod was planning a resistance against Rome and so on. Eaten by worms, it is something that happens to us in death. For it to happen in life, to be eaten alive by worms infesting your wounds and abscesses is a bit more than painful. Luke says an angel struck him down because he didn’t give God the glory. He a king of the Jews was setting himself up to be a god in the manner of so many eastern and Roman tyrants. So Herod dies, who had started a persecution of the church, and this paves the way for the word of God to increase and multiply.
Luke gives the credit of growth in the church to the word of God alone. It increases and multiplies, not the church, not the people but the word of God.
And all of this seems to happen while Saul and Barnabas are in Jerusalem on behalf of Antioch delivering the coin they had gathered for relief of the poor. As they return to Antioch, they bring John Mark with them.