Lu Ten's death had a traumatic impact upon his father, causing him to abandon the siege, ultimately providing an opportunity for Ozai to usurp the birthright of Fire Lord from his brother.
Lu Ten lost his life in combat against the Earth Kingdom five years before the end of the Hundred Year War during the Siege of Ba Sing Se. When he learned of his son's death, Iroh was so heartbroken that he abandoned the siege of the city, claiming that he and his men were exhausted from the extensive time away from home. When his brother, Prince Ozai, learned of Lu Ten's death and Iroh's subsequent actions, he petitioned their father, Fire Lord Azulon, to appoint him as heir to the throne, claiming that Lu Ten's death meant that the royal bloodline would end with Iroh, since he had no other heir. Azulon was enraged at such a request, but because of his mysterious death soon after, Ozai was able to take the throne. Iroh did not challenge Ozai's claim, even though he was the elder brother, and did not attempt to secure the throne for himself.
Lu Ten and Iroh had a strong relationship during Lu Ten's childhood, and they remained close during his adulthood. His death had a profound effect upon Iroh, whose personality shifted more toward a calm acceptance of life, rather than continuing to participate in the current war.
Lu Ten was first mentioned during the Siege of the North when Iroh informed Zuko that after Lu Ten's death, he began treating Zuko like his own son. A younger Lu Ten was also seen in "Bitter Work" and a picture of an older Lu Ten was displayed at Iroh's altar for him during "Tales of Ba Sing Se". Zuko had a flashback showing himself, Iroh, and a young Lu Ten during "The Beach".