Reign

Reign is a Worldkiller, a biological weapon created on Krypton but eventually outlawed by the Kryptonian Science Council.

After Kara awoke on Earth, the sunstone that was in her pod led her into space and to an abandoned Argo Citythe only remaining remnant of Krypton. While there, she learned the truth about her home world and the death of her parents before finding herself confronted by Reign.

As you can likely deduce, Reign wasnt Kryptonian, but she was a product of Krypton. She was what they called a Worldkiller, a mysterious and feared weapon built far off-world by Kryptonian scientists that was eventually deemed too terrible and destructive for actual use. Placed in stasis and never brought down to Krypton, Reign awoke with no real memory of her life prior to awakening. She came to Argo City and eventually Earth searching for answers about her past.