A demon who had received the boon from Brahma that he could only be killed by the earth. The earth was his mother, a mother would never kill her child. Naraka became a plague.
Naraka is also called Bhauma or Bhaumasura because his mother was Bhumi, the earth.
Satyabhama, a wife of Krishna, the embodiment of the earth, killed him. Krishna freed 16,000 women from Naraka’s captivity. They didn’t want to go back to their families, they wanted to stay with Krishna. He married them all.
Read about Naraka here