Final answer:
The phrase "Great Happiness" is mentioned by King Duncan in Shakespeare's play Macbeth, reflecting his joy at the victory in the war.
Step-by-step explanation:
The phrase "Great Happiness" is said by Duncan in the play Macbeth. The context of this line is that King Duncan expresses his joy at the victorious outcome of the war, acknowledging Macbeth's brave contribution to the battle that resulted in their win. The full line spoken by Duncan as mentioned in the text provided is: "LibreTexts™ Curbing his lavish spirit: and, to conclude, The victory fell on us. Duncan. Great happiness!". This display of happiness signifies Duncan's satisfaction with Macbeth's loyalty and valor.