One day, Narada visited the Pandavas and they received him respectfully and with great honour. He observed the prosperity of the kingdom and the unity of the brothers.
He also observed their unique marriage and understood that this arrangement could someday become dangerous.
Later, Narada warned them that even loving brothers could become enemies if rivalry over a woman arose, and in order to explain this statement, he narrated the story of Sunda and Upasunda.
Sunda and Upasunda were two asura brothers who were deeply loyal to one another and were inseparable. The brothers ruled together and shared everything.
They performed severe austerities and pleased Lord Brahma, who granted them a boon. The brothers cleverly asked that no being in existence should be able to kill them except each other.
After receiving the boon, the brothers became arrogant and disturbed yajnas. The devas, fearing the asura brothers, approached Lord Brahma, who ordered Vishvakarma to create a beautiful apsara named Tilottama.