Mark 6:17

For Herod himself had sent forth
Some of his guard, a detachment of soldiers,

and laid hold upon John;
who seized upon him, and took him up:

and bound him in prison;
in the castle of Machaerus:

for Herodias' sake, his brother Philip's wife, for he had married
her;
whilst his brother was living, and who had had children by her; (See Gill on Matthew 14:3).