Kamsa became so cruel that he just did not care about anything anymore. He already had the protection of his father-in-law - Jarasandha's army. He also genuinely believed that if he killed the children of Devaki, he could conquer his own fate.
That was when news reached him that Devaki was carrying the seventh child.
Kamsa was ready to kill this child too when he had some strange news – Devaki aborted the seventh child.
Unknown to Kamsa, Rohini - another wife of Vasudeva was carrying a child. Rohini was living with Nanda, Vasudeva's brother in the village of Gokul, presumably to escape persecution from Kamsa.
(It was said that this abortion was primarily done to confuse Kamsa.
When Devaki aborted the seventh child, it would be unclear whether the next child to be born would be considered as the seventh or the eighth child.
More importantly, this is the first time that Goddess Yoga Maya is introduced in the Bhagawat Purana.
Yoga Maya is the source of all the powers of Lord Vishnu. She is supposed to be Vishnu's power.
It is believed that the seventh child of Devaki had not been aborted. Instead Yoga Maya transferred the child from the womb of Devaki to the womb of Rohini.
This child of Rohini was Balarama – Krishna's elder brother.
Some Puranas consider Balarama as an avatar of Lord Vishnu. Some other Puranas consider Balarama as the incarnation of Sesha – the thousand headed snake on whom Lord Vishnu rests.)
When news came to Kamsa that Devaki was carrying the next child, he did something unforgivable. This time he imprisoned both Devaki and Vasudeva.
Somehow Kamsa was sure that this was the child – the one who would be his end. The reason Kamsa was so sure that this child was meant to his death was that Devaki had almost started glowing. (The Bhagawat Purana gives a brilliant description of how Devaki looked at this time.)
Kamsa never knew it, but this was the time Nanda, Vasudeva's brother also had some good news. His wife Yashoda was carrying a child.
There is a rather interesting description in the Bhagawat Purana at this time.
It states that Kamsa was very worried as he saw Devaki. Kamsa was so anxious that he actually wanted to kill Devaki. But he stopped himself saying that Devaki was his sister and that if he killed Devaki now, it could lead to disrepute for him.
(The Purana already mention that Kamsa ruled like a tyrant and that people were afraid of him. It is curious as to why Kamsa would be so worried about his reputation at this time.)
So in a prison, with strict guards standing vigil outside, with glorious flooding rain lashing through Mathura, Devaki gave birth to her eighth son – a beautiful dark complexioned baby – the baby was said to be so dark that he seemed to give out an almost bluish tint.
(It was supposed to be an extraordinarily auspicious time for birth. And it was not just the eighth child of Devaki which was born at this time. Yogamaya was born as Yashoda's daughter at the same time. Not many know it, but Arjuna the third Pandava, another hero of Mahabharatha was also born on the same time as the eighth child of Devaki.)
(There is another very interesting anecdote about this event.
Devaki and Vasudeva were great sages during their previous births. They prayed to Lord Vishnu with great devotion.
When Lord Vishnu appeared before them, both wanted a son equivalent to Lord Vishnu himself.
They were so enthusiastic about it, that they asked the same boon three times. Lord Vishnu is said to have granted their boon.
And because of this, Lord Vishnu was born three times as the son of the couple.
During the Satya Yuga (the first yuga), there lived a King called Sutapas and this wife Prushni who were the incarnation of Vasudeva and Devaki. Lord Vishnu was born as Prushnigarbhan (meaning one who is in the womb of Prushni).
During the Treta Yuga (the second yuga), Vasudeva and Devaki incarnated as Sage Kashyap and Aditi. Lord Vishnu was born as Aditi's son – Upendra.
In this avatar, Lord Vishnu was a dwarf so he is called as "Vamana". (Literally translated Vamana means dwarf.)
The Vamana Avatar is an extraordinarily interesting incarnation of Lord Vishnu.
During this time, Bali the asura ruled the world. He was a brilliant ruler – just and kind.
But he was an asura.
Naturally, Indra and the other Devas could not tolerate it. They prayed to Lord Vishnu to get rid of Bali.
Lord Vishnu is said to have heard their prayers and Lord Vishnu incarnated as Vamana, the dwarf.
Vamana then went to Bali who was giving away alms.
Vamana asked for three feet of land to be measured by his own foot.
Unthinkingly, Bali promised to give Vamana what he had asked for.
That was when Vamana grew to a huge size (This is called as Trivikrama Avatar of Lord Vishnu)
Vamana grew so large that he covered the entire earth in one step, the skies in the other.
Vamana then asked Bali for his third step and that was when Bali offered his own head as the third step.
Vamana supposedly pushed Bali to the netherworld with the third step.
But Lord Vishnu was so pleased with Bali that he promised the asura that he was going to be the next "Indra" during one of the coming eras.
And keeping his promise to Vasudeva and Devaki, Lord Vishnu was born for the third time as Devaki's eighth son.)