Duncan is the king of Scotland, father of Malcolm and Donalbain, and the victim of the crime that drives the play. He is a generous and trusting king, and he dies in the house of the man he himself honored.
In Act I, scene 2, he condemns the traitor Thane of Cawdor to death and gives the title to Macbeth: what Cawdor lost, the noble Macbeth has gained. In scene 4, on hearing of Cawdor's repentant death, he says that no art can read a man's character in his face, and that he was a gentleman in whom he placed absolute trust. At that moment Macbeth enters, and Duncan tells him: "I have begun to plant you, and I will do all I can to see you grow tall and flourishing." Then he names Malcolm Prince of Cumberland and announces that he will go to Inverness, Macbeth's castle.
In scene 6, he praises the place ("This castle has a very pleasant setting") and is received by Lady Macbeth. In scene 7, Macbeth himself acknowledges the king's qualities: Duncan has borne his office with such meekness. He is killed at night, offstage, and Macduff discovers the body in Act II, scene 3.
The death has effects beyond the castle. Ross and an Old Man tell that the day seemed like night and that Duncan's horses, loose, devoured each other. The body is taken to Colmekill, the resting place of earlier kings, and in Act IV Macduff reminds Malcolm that his father was a saintly king.