Who were those 7 sons of Devki and Vasudev killed by Kans, the king of Mathura before the birth of Krishna?
In the book Sri Hari-vamsa (2nd chapter of the Visnu-parva) it is written that Hamsa, Suvikrama, Kratha, Damana, Ripu-mardana and Krodha-hanta, who are known as the sad-garbha (six unborn children), are the sons of the demon Kalanemi. They performed intense austerities to worship the universal grandsire Brahma, unbeknown to their own grandfather Hiranyakasipu. Brahma, being satisfied with their extreme penances, granted them the boon of protection from death according to their prayers. Later, when Hiranyakasipu heard about this incident, he became enraged and said, “You have worshipped Brahma without letting me know, thus I have no affection for impudent people such as you. Your father himself will kill all of you. In your next life all six of you will take birth in the womb of Devaki and your father Kalanemi will be born as Kamsa. That very Kamsa will be your murderer.” Due to this curse of Hiranyakasipu they took birth in the womb of Devaki and were killed by Kamsa, the incarnation of Kalanemi.
Prior to this, they were well known by the names Smara etc. and were the sons of Marici Muni. They laughed after seeing the universal grandsire Brahma running after his own daughter. Due to this offence of looking down on a great personality, they had to take birth as demons.
In the Srimad Bhagavata (10.85.47-57) it is mentioned that due to the prayer of Mother Devaki, Sri Rama and Sri Krsna brought back her six dead sons from Sri Baliraja’s Sutala-puri:
marice sat putra urnayam prathame’ntare
devah kam jahasur viksya sutam yabhitum udyatam
tenasurim agan yonim adhunavadya-karmana
hiranyakasipor jata nitas te yoga-mayaya
devakya udare jata rajan kamsa-vihimsitah
sa tan socaty atmajan svams ta ime’dhyasate’ntike
itah etan pranesyamah matri soka apanuttaye
tatah sapat vinirmuktah lokam yasyanti vijjvarah
smarah udgithah parisvangah patangah ksudrabhrt ghrni
sadime mat prasadena punah yasyanti sat gatim
This means that after going to Sutala-puri Sri Krsna told Bali, the king of the Daityas, “O lord of the Daityas, during the manvantara of Svayambhuva, six demigod-like sons were born in the womb of Urna-devi, the wife of Maharsi Marici. Seeing Brahma running after his own daughter (namely Sarasvati) they mocked him. As a result of this sin – hiranyakasipu saksat kalanemi-ksetre – they took demoniac births as the sons of Kalanemi who was the son of Hiranyakasipu. Again from there they were forced to take birth in the womb of Devaki through Yogamaya and killed by Kamsa. Mother Devaki is mourning for them, considering them to be her own sons. Presently they are residing in your abode. To alleviate her maternal sorrow I will take them away from this place to meet their mother. Finally, they will go back to the planets of the demigods having been freed from their curse and from lamentation. These six sons of Marici namely Smara, Udgitha, Parisvanga, Patanga, Ksudrabhrta and Ghrni will eventually attain the Supreme Abode by My mercy.”
The seventh child of Devaki and Vasudeva was Balram, who was the incarnation of Sheshnag. Vasudeva prayed hard to Lord Vishnu to save this child. Then, by the power of Lord Vishnu's Yoga Maya, the child was secretly transformed into the womb of Rohini, another wife of Vasudeva (she lived in Vrindavan), while everyone thought that she had lost that child due to miscarriage. Soon Rohini gave birth to the child and it was grown in the village of Vrindavan.
