The Door of Truth was a mystical, talking door which blocked the passage to the second level of the Temple of the Sun. It asked a question of all who would pass through it. When Connor approached it demanded: "Speak, thou, of Order." Connor responded, "Order is Unity, and therefore Perfect Harmony."
Satisfied, the Door of Truth opened.
The Door of Truth is a puzzle presented near the end of King's Quest: Mask of Eternity.