Complex sentence: Once he finds them, they show him a vision of slain soldiers who tell him to beware Macduff, and he shall be King until Great Birnam Wood comes to Dunsinane Hill, where the royal castle is built.
Split into simpler sentences: Once he finds them, they show him a vision of slain soldiers who offer him predictions and warnings. He must beware Macduff, and he shall be King until Great Birnam Wood comes to Dunsinane Hill, where the royal castle is built.