At Dyutasabha, Duryodhana bared his thigh and gestured to Draupadi to come sit in his lap. Bhima was furious at this and he vowed to break Duryodhana’s things in battle to avenge the insult to Draupadi. At Kurukshetra, that is how Duryodhana died, after having his thighs broken by Bhima.

 
But, even before the final battle, Duryodhana earned a curse too that promised the same consequence. This happened during the Pandava vanvaas.

Dhritarashtra was both happy and fearful that the Pandavas had gone away into exile. He was happy because his son now had control over Indraprastha but he was afraid because he knew of the Pandavas’ strength and abilities.

Once, Rishi Maitreya visited Hastinapur. Dhritarashtra anxiously asked him about the Pandavas and whether there was a possibility of things worsening. 

Rishivar told him that the insult to the Pandavas should never have happened and that he should have stopped it. He warned that destruction was inevitable if the situation was not corrected immediately.

Turning to Duryodhana, who was also in the sabha, the Rishi tried to tell him to make peace with the Pandavas too. But arrogant Duryodhana was not pleased at all. With a smirk on his lips, he slapped his thigh repeatedly as if challenging the Rishi but did not speak a word.

The Rishi was furious with Duryodhana’s insolent response and he cursed that Bhima would smash his thigh in battle. The blind king Dhritarashtra was alarmed and he begged the Rishi to take back his curse. But it was not to be. The Rishi warned him that if Duryodhana did not make peace with his cousins, nothing would stop the curse from coming true.    

So it was not just Bhima’s vow but also Maitreya rishi’s curse that brought about Duryodhana’s end with his thigh being broken.