Closely related to the social concerns of The Hollow Hills is its main theme and indeed the main theme of the entire Merlin-Arthur story: man's free choice and reconciliation to his destiny. Merlin in The Crystal Cave has the choice between manhood and power, chooses power, and spends the rest of his life helping his second-self — Arthur — achieve power over the nation. Arthur in The Hollow Hills follows his destiny to become the warrior king but sows the seed of his own death when he sleeps with his half sister Morgause, impregnating her with Mordred. Mordred in The Wicked Day (1983) knows of the prophecy that he will bring about his father's death, loves and serves his father, yet becomes enraptured by power and trapped by circumstance into living out the prophecy. Stewart thus seems to believe that man has some measure of free will but that it carries awesome responsibility and always works in keeping with the greater will of God, destiny, or fate.