Mark 6:18

For John had said Herod was probably on his way to meet his father-in-law, when he first encountered the Baptist, who, in the presence of the Galilean king, proved himself no "reed shaken by the wind" (Luke 7:24), but boldly denounced the royal crimes (Luke 3:19), and declared the marriage unlawful. For this outspoken faithfulness he was flung into prison, probably in the castle of Machærus or "the Black Fortress," which Herod's father had built in one of the most abrupt wâdys to the east of the Dead Sea, to overawe the wild Arab tribes of the neighbourhood. Though originally in the possession of Aretas, Herod had probably seized the fortress after the departure of his first wife to her father's stronghold at Petra (Jos. Ant. xviii. 5. 2).