The pilgrimage of Soma

After this, Balarama went to the pilgrim place of Soma. He had a bath there and gave gifts of cows to the Brahmanas. 

Janmajeya asked the sage about this place. The sage said - 

Once Shiva was making love to Durga. The sperm of Shiva was burning and Durga unable to control that, flung that in the waters of Ganga. 

Ganga herself was not able to stand this and left the sperm in the clump of grass on the banks. From that was born a boy with six heads. Six wives of Chandra, seeing the boy, came to take care of him. They fed the boy and named his Kartikeya as the first one who had fed him him was Kritika. 

The son of Shiva grew up. Indra asked Shiva to get the maiden Devasena get married to Kartikeya. He would then become the husband of Devasena (Devasenapati) and would become the commander of the Gods and thus eventually would kill Tarakasura.

Shiva agreed to this. 

In the war with Tarakasura, Indra lost the battle and came to Kartikeya for help. All Gods gave weapons to Kartikeya and with those, Kartikeya killed Tarakasura. 

The sage said to Janmajeya - Here Balarama also heard the story of the birth of Kartikeya and then proceeded further.