The eldest Kaurava, whose refusal to grant the Pandavas even five villages makes him the Mahabharata's chief antagonist and the war's proximate cause.
महाभारत · Mahabharata
Duryodhana's rivalry with his Pandava cousins begins in childhood and deepens through the humiliations of the rigged dice game he orchestrates with his uncle Shakuni, stripping the Pandavas of their kingdom, dignity, and freedom for thirteen years. Even after their exile is honored in full, he refuses Krishna's final peace mission, rejecting even a token settlement of five villages for the five brothers.
Duryodhana's death, his thigh broken by Bhima in a blow considered to break the rules of fair mace combat, closes the war's main fighting; tradition holds that Bhima's vow to break that same thigh dates back to Duryodhana's own humiliation of Draupadi, tying the war's beginning and end to the same unresolved grievance.