When Vaivaswat Manu did not have any children after many years of marriage, he decided to consult Vashishth muni. Manu along with his wife Shraddha conducted a yajna to please twin Gods Mitra-Varun.

At the end of the yajna they had a daughter, whom they named Ila, since Shraddha secretly wished for a daughter. However, Manu wanted a son and he requested Vashishth rishi to do something about it.

Vashishth muni turned Ila into a man upon insistence from her father and he was called Sudyumna.
The fate had other plans though. One day Sudyumna was hunting near Sumeru mountain and he entered a grove following a deer.

That grove was private abode of Mata Parvati and in order to protect her privacy Bhagwaan Shiv had enchanted that grove in a manner that anyone entering it would become a woman.

Sudyumna, who had now become Ila again, requested Bhagwaan Shiv and Mata Parvati to reverse the enchantment, so they allowed him/her to switch between male and female every month.

Once Ila was roaming around and Chandradev’s son Budh saw her and proposed to marry her. Both married and lived happily for a month. At the end of the month Ila turned into Sudyumna (Ila) and did not remember anything.

Budh convinced Sudyumna (Ila) to stay with him for a year and later Ila gave birth to Pururava, who started the Chandravansh dynasty.

In this image Budh is trying to convince Sudyumna to stay with him.

Founder of Suryavansh, Ikshwaku was Ila's brother.