The Birth: Almost 250,000 years ago, on a planet with an environment so hostile that it seemed incapable of supporting any life, there existed a team of scientists who were bent on creating the ultimate form of life. The lab was under a secure dome with proper security systems, shielding away the harsh temperatures and hostile monsters of the planet. Their experiment stsrted out by subjecting a baby to the external environment. The heat burned the child while the beasts tore apart it's flesh. The scientists, under the leadership of Bertron gathered the remains of the baby and cloned a brand new infant. - This was the begining of the creature called Doomsday.
Everytime a new infant was cloned from the previous baby's remains it was subjected to the external environment, thereby conditioning it's body to the environment. The beasts would ultimately tear the infact to shreds and the process would begin again.
Bertron's baby started to evolve. At first, the atmosphere killed him in seconds; this was gradually stretched to minutes. After more than thirty years of experimentation, the infant was no longer a baby - Bertron called him the Ultimate. The rest of the universe would call him Doomsday!
Doomsday's debut was in Superman: Man Of Steel #18 (December,1992)

The Legend: The Ultimate was able to survive outside the lab for almost two years. As for the hostile beasts of the planet; he had hunted them to extinction. He had defeated death!
The Ultimate was beyond the normal conventional death. He did not need to eat or breathe. He was nourished by solar energy which he could store in his body for a large period of time. His body was of pure flesh and he was deviod of any internal organs. The creature could adapt to any environment. If he would indeed be killed by another, he would return to life as an evolved being, rendering the previous method used to kill him, useless. He was a mindless creature that seemed to live only for destruction.
The Ultimate finally invaded the lab and killed the entire team of scientists who had "birthed" him. The events of his past had gone beyond memory and became a part of his genetic code. He was programmed to kill anything he considered hostile. His genetic makeup had recorded the fact that the scietists had killed him a thousand times before. That's why he dealt the killing blow to them.
The creature escaped on one of Bertron's ships and landed on another planet and dealt the same fate to those inhabitants. He kept on hopping from planet to planet spreading destruction everywhere he went; thereby earning the name : Doomsday.
By the time Doomsday arrived on Calaton, he had already gained the reputation of being an instoppable killing force. The rulers watched helplessly as Doomsday wreak havoc on their planet. It was now their time to act. The rulers of Calaton had special metahuman powers which they willingly gave up along with their lives to create The Radiant, a being of pure energy. Radiant defeated Doomsday, but the body was never destroyed; instead they cast the body into space where it drifted forever, only to come to life again ...in Metropolis!
Superman fought Doomsday to a standstill where both the being were killed. Superman(See Superman) came back to life and so did Doomsday. Unlike Superman though, Doomsday had evolved and come back stronger than before.
Doomsday's body had been thrown into space by the rogue Cyborg (See Cyborg) and the creature was brought to Apokolips (See Darksied) by unsuspecting traders. Around this time, Superman felt a need to find the creature who had killed him and followed his trail to Apokolips. But by then, he had already been sent to Calaton, the planet which first defeated him. Once again, the Radiant was created to destroy him. This time though Doomsday knew how o tackle the Radiant and eventuallt killed him. Superman hunted him down and with Waverider's help he finally deated the creature.
Doomsday came back once again to fight Superman, only this time he was being controlled by Brainiac, a demolished being from the planet Colu.(See Brainiac)

Man Of Steel #18