David's son who defiled his sister is known as _________________.

The answer is Amnon, who is the son of King David and is known for the grievous act of defiling his half-sister, Tamar. This incident is detailed in 2 Samuel 13, where Amnon became infatuated with Tamar and ultimately raped her. This act set off a tragic sequence of events within David's family, leading to further conflicts and the eventual estrangement between Amnon and his brother Absalom, Tamar's full brother, who sought revenge for the assault.

The significance of Amnon's actions lies in their consequences, reflecting themes of sin, family strife, and the tragic outcomes of unchecked desires, which are prevalent in the narrative of David's life. This event is pivotal in illustrating the moral decay within David's household and sets into motion several key plot developments in the history of the Israelite monarchy.