Rishi Maitreya came to Hastinaapur to warn Dhritraashtra about what he did to his younger brother's sons, but Dhritraashtra did not agree with him, so Maitreya Muni gave him Shaap that whatever vows Bheem took would surely come true - means that he will lose his all sons killed by Bheem, Bheem will drink he blood of Dushaasan's chest; and he will break Duryodhan's thigh.