In the Satya Yuga, Diti and Sage Kashyap had two powerful sons - Hiranyaksha and Hiranyakasipu.
Unfortunately, both the brothers were very ambitious and very blood thirsty.
First, Hiranyaksha performed severe penance towards the Gods and became extraordinarily powerful. After getting these powers, Hiranyaksha then took over the Skies and had the Devas thrown out of there. Hiranyaksha ruled like a dictator and he killed anyone who stood up against his rule.
Therefore to protect earth and the other realms from Hiranyaksha, Lord Vishnu incarnated as a Varaha (Varaha in Sanskrit means a "Boar") and killed Hiranyaksha.
The world was free from the tyranny of Hiranyaksha. The Devas came back to the Skies and the people everywhere rejoiced. All except for one man - Hiranyakasipu.
Once Hiranyakasipu learnt about how Lord Vishnu had killed his brother, Hiranyakasipu was filled with blind hatred. Hiranyakasipu promised himself that he would complete his brother's undone work and this time do it more ruthlessly and swiftly.
Hiranyakasipu set about performing penance towards Lord Brahma for several years and after that HIranyakasipu obtained an impossible boon from Lord Brahma. The boon that Hiranyakasipu obtained made him near impossible to kill. Because Hiranyakasipu prayed that he could not be killed by any creature created by Brahma and that he should not be within or outside any residence while he was killed or during daytime or night or in the ground or the sky. [Lord Vishnu incarnated as "Narasimha" - which means "Half Man - Half Lion" and also fulfilled the other conditions to kill Hiranyakasipu.]
Before Lord Vishnu incarnated as the Narasimha, however, Hiranyakasipu followed his brother's footsteps and ruled like a tyrant. People everywhere were scared of Hiranyakasipu as he took over the throne of Indra, and killed people mercilessly. In fact it went so bad that Hiranyakasipu even killed people who worshiped any of the Gods.
It was during this time that there was another asura in the court of Hiranyakasipu called Kalanemi. Kalanemi had six sons - Hamsa, Suvikrama, Kratha, Damana, Ripumardhana and Krodhahantha who were collectively called as the Sada Garbhas.
While Kalanemi supported Hiranyakasipu's conquests, the Sada Garbhas opposed Hiranyakasipu. In fact, the Sada Garbhas were extremely devoted to Lord Vishnu.
Ignoring Hiranyakasipu's teachings of disregarding the Gods, the Sada-Garbhas prayed to Lord Brahma.
They performed severe penances and austerities and finally pleased Lord Brahma appeared before them.
When Lord Brahma appeared, he was so pleased with the Sada Garbhas that he was pleased to grant them any boon of their choice, "I know of your pious nature and I am very pleased with all of you. Please ask me any boon," The Creator God told the Sada Garbhas.
The Sada Garbhas made Brahma even more glad by the boon that they asked him. "Please grant us a boon that we would not be hurt by any humans, animals or divine or the Demi-Gods! And if you are very pleased with us, grant us the boon that even the great sages would not be able to hurt us with their curses!"
Lord Brahma was completely pleased with the pious Sada-Garbhas and happily granted the boon to the six of them.
However the Sada-Garbhas were in a world of trouble.
When Hiranyakasipu heard of the boon that the Sada Garbhas had from Lord Brahma he was very angry with them, "You approached Lord Brahma?" Hiranyakasipu fumed angrily at the six Sada Garbhas, "And you wanted protection!" Hiranyakasipu said in a disgusted voice.
"I am the only God you should worship. Not Brahma or Vishnu!" Hiranyakasipu thundered and angrily cursed all of them, "You think Brahma's boon will protect you!' He sneered. "I curse you that all of you will be killed by your own father!"