Muchukunda

Everybody knows that Shri Rama was born in the Ikshavaku dynasty, the descendant of Suryavansha. The most famous king Mandhata came from the same lineage.

Challenged Indra

http://ayomed.blogspot.com/search/label/Born

King Muchukunda was his son. His kingdom included the entire length and breadth of the earth. He was so powerful that even Indra used to be desirous of his help.

http://2twi.blogspot.com/search/label/Muchukunda

Helped Indra
https://www.google.com/search?q=Emerald+Lingam
http://6tri.blogspot.com/2012/10/somaskanda-seven-places-photos-location.html

River
https://www.youtube.com/results?search_query=muchukunda+river