Gandhari's Gaze and Duryodhan's Downfall

The Diamond Armor: Gandhari's Gaze

Before his final battle with Bheem, Gandhari instructed Duryodhan to bathe in the Saraswati and come to her naked. Gandhari's eyes were so powerful that if she were to look at her son, his body would become as hard as a diamond.

Krishna's Ruse: The Exposed Weakness of Duryodhan

While Duryodhan was going naked to his mother, he met Krishna.

"Don't you have any shame going to your mother naked like this?" Krishna jeered at Duryodhan. "At least cover your genitals with your hands." 

Duryodhan fell for Krishna's trick and covered a part of the area beneath his waist with banana leaves.

The Chink in Duryodhan's Armor: A Mother's Incomplete Blessing

Gandhari removed the cloth covering her eyes to look at Duryodhan. She saw that Duryodhan was not completely naked. The parts of Duryodhan's body where her sight fell became hard as iron. But a region below the waist was left out. During Duryodhan’s battle with Bheem, the latter took advantage of this and severely wounded him.