Hansa and Dimvaka

krishna to Yudhistira

After the immediate cause of fear was removed (by the death of Kansa), Jarasandha, his father-in-law, took up arms. Ourselves consisting of the eighteen younger branches of the Yadavas arrived at the conclusion that even if we struck our enemies continually with excellent weapons capable of taking the lives of the foes, we should still be unable to do anything unto him even in three hundred years.

As Jarasandha hath with him

1. two friends that are like unto the immortals, and 
2. in point of strength the foremost of all men endued with might. 

They are called Hansa and Dimvaka who are both incapable of being slain by weapons. 

1. The mighty Jarasandha, being united with them, becomes incapable, I think, of being vanquished by even the three worlds. 
2. O thou foremost of all intelligent men, this is not our opinion alone but all other kings also are of the same mind. 

There lived, O monarch, a Another king named Hansa, who was slain by Rama (Valadeva) after a battle of eighteen days.

But, O Bharata, 

1. hearing people say that Hansa had been killed, Dimvaka, O king, misunderstood that his friend Hansa had been killed , he jumped into the waters of the Yamuna. 
2. Afterwards when Real Hansa, the subjugator of hostile heroes, heard that Dimvaka, had died, he also went to the Yamuna and jumped into its waters. 

Then, O bull of the Bharata race, king Jarasandha, hearing that both Hansa and Dimvaka had been Died, returned to his kingdom with an empty heart. 

http://www.sacred-texts.com/hin/m02/m02014.htm
https://www.google.com/search?q=hamsa+and+dimbaka