Final answer:
Akhenaten introduced monotheism during the New Kingdom by worshiping the sun disk, Aten, but it was short-lived as Egypt returned to polytheism after his death.
Step-by-step explanation:
The pharaoh who introduced monotheism during the New Kingdom of ancient Egypt was Akhenaten, known originally as Amenhotep IV. Akhenaten established a monotheistic belief system centered around the worship of the sun disk, Aten, which represented a significant departure from the traditional polytheistic religion that worshipped a pantheon of gods. His reign, which lasted for 17 years from approximately 1353-1336 BCE, saw the temporary establishment of monotheism in Egypt, but after his death, the traditional religious practices were largely restored by his successors.