Doomsday first appeared in Superman: Man of Steel #17 (Nov. 1992).
Doomsday is best-known as the character who killed Superman.
Originally known as "The Ultimate," Doomsday was born in prehistoric times on Krypton, long before the humanoid Kryptonian race gained dominance over the planet about 250,000 years ago. It was a violent, hellish world where only the absolute strongest of creatures could survive. In a cruel experiment involving evolution, intended to create the perfect living being, the alien scientist Bertron released a humanoid infant (born in vitro in a lab) onto the surface of the planet, where he was promptly killed by the harsh environment. The baby's remains were collected and used to clone a stronger version. This process was repeated over and over for decades as a form of accelerated natural evolution. The agony of these repeated deaths was recorded in his genes, driving the creature to hate all life.
As it evolved, the child eventually became able to survive the high temperatures and searing atmosphere, only to be quickly slain by the vicious predators that inhabited the planet. Over time, and without the assistance of Bertron's technology, he gained the ability to thrive on solar energy without the need for food or air, to return to life and adapt in order to overcome whatever had previously killed him. The Ultimate hunted and exterminated the dangerous predators of Krypton. He then killed Bertron himself, whom he had come to identify as an enemy.
The Ultimate escaped Krypton via a ship that regularly arrived to deliver supplies to Bertron (who had wanted little contact with the planet's natives) and went on a killing spree across several planets. It began 245,000 years earlier on Bylan 5, where Darkseid was about to wed a princess (in order to obtain that planet's chemical deposits for Apokolips' weapons factories). Just as the Ultimate and Darkseid were about to meet in combat, Darkseid was forced to flee, as the rampage of The Ultimate had caused the planet's atmosphere to become toxic, thereby rendering the chemicals worthless to Apokolips. The Ultimate hitched a ride on an escaping shuttle, which crashed on Khundia. The warring Khundian clans united in order to build protective armor for a warrior named Kobald, who they hoped would survive long enough to force the Ultimate onto a rocket. Once the rocket was in space, the Ultimate killed Kobald and the resulting explosion sent him hurling through space.
He next crossed paths with a Green Lantern named Zharan Pel. The Ultimate took the Lantern's power ring and, sensing the power of the Guardians of the Universe, headed toward them. The hundreds of Green Lanterns, which were sent to stop him, were slaughtered. He continued to Oa, where a single Guardian sacrificed himself in battle to defeat him. The release of energies by the Guardian caused a tear in space through which the Ultimate fell.
Eventually arriving on the planet Calaton, he ravaged that world for three years. With only the capital city left, the royal family combined their life forces into a single energy being, the Radiant. The Radiant killed the Ultimate with a huge blast of energy (laying waste to over a fifth of his planet in the process). In common Calatonian burial procedures, the Ultimate's seemingly dead body was suited and shackled to prevent his spirit from escaping into the afterlife, and he was shot into space because the murders he committed made him unworthy of burial on Calaton. Eventually, his metallic casket crashed on Earth, the force of the impact driving it deep underground.
After freeing one arm and smashing his way out of his buried vault, The Ultimate went on a rampage in Midwestern America, where he first encountered the Justice League. He defeated the entire team of superheroes in a matter of minutes, which in turn attracted the attention of Superman. Most notable is the fact that the creature fought the whole time with literally one hand tied behind his back (due to his ancient burial cables), yet was still able to lay waste to all opposition and much of the surrounding area. The only Justice Leaguer who could defend herself against the creature was Maxima. The creature gained its new name when League member Booster Gold remarked how the rampage resembled "the arrival of Doomsday". The comment reached the broadcast media and led to the creature becoming known by its new name of Doomsday. Five Leaguers, including Superman, combined their energy powers in an attempt to take Doomsday down, but succeeded only in destroying the last of his ancient burial cables, allowing him to use both hands.
During his rampage, Doomsday's interest was captured by billboards and television spots advertising violent wrestling competitions held in Metropolis, which appealed to his blood lust and thus enticed the otherwise mindless creature to head towards the city. In counterattacking the creature, Superman quickly found that his opponent's power was a match for his own, and he realized that if Doomsday actually reached Metropolis, the resulting battle could conceivably destroy the city and kill millions of innocent people. Doomsday developed a strong desire to murder Superman, a desire that was later explained in the Hunter/Prey miniseries: from the agony of continually dying during his creation process, Doomsday developed in his genes the ability to sense anyone Kryptonian, as well as an overriding instinct to treat any such being as an automatic threat.
In the space of a few issues of the Superman comic book series, Doomsday battled Superman in a titanic struggle, leading the hero to conclude that the creature would continue to attack relentlessly and endlessly, with no fear or compassion. Matters came to a head in Superman (vol. 2) #75, wherein both Doomsday and Superman beat each other to death in front of the Daily Planet building in Metropolis.
In the aftermath of Superman's apparent death, four super-beings appeared in his wake, two of them declaring themselves to be the "real" Superman. One of these four, a half-man/half-machine who greatly resembled Superman with cybernetic implants where he had sustained the greatest amount of damage from Doomsday's blows, took custody of Doomsday's apparently lifeless body. After strapping the creature to an asteroid with an electronic device attached, the cyborg flung Doomsday into deep space, on a trajectory supposedly certain to never intersect any other planet. The issue ended with a frame of a reawakened and laughing Doomsday, still chained to the asteroid but otherwise alive.
Eventually, after passing through a wormhole, Doomsday's asteroid was accidentally found by a deep-space scavenger ship. Upon closer examination of the peculiar-looking drifting rock, the ship's crew retrieved the object, hoping to find something of value. The scavenger vessel happened to be on a route to Apokolips, the home of the powerful tyrant Darkseid. Doomsday was fully rested and, after slaughtering the crew of the salvage ship, found himself landing on the harsh world. This was to be the setup for a final showdown between Doomsday and Superman, who had been uneasy about the possibility of Doomsday's resurrection. With the help of his JLA contacts, Superman procured a Mother Box, a sentient computer, after Darkseid's servant Desaad contacted Earth about a problem on Apokolips.
Unknown to Superman, Doomsday had faced and beaten Darkseid in single combat, even after withstanding the full effect of Darkseid's fabled Omega Beams, and was laying waste to Apokolips. Unfortunately, before Superman could deal with Doomsday, Desaad opened a boom tube to Calaton—the first world where Doomsday was successfully defeated—and sent Doomsday through, to what he believed was his defeat at the hands of the Radiant. However, Doomsday was able to adapt and overcome any opponent because of the process by which he was created, so, although the Radiant had defeated him once, he would not be able to defeat him again. Likewise, even though Superman had killed Doomsday once before, he was unable to do so again. Superman, while knowing this — having been filled in on Doomsday's history by the time-manipulating Waverider — was obsessed with stopping Doomsday and followed him to Calaton. He fought Doomsday again with the help of the Mother Box, but, despite it providing him with extra weapons such as an ultrasonic gun and an energy sword, Superman met with defeat. He was forced to use one of Waverider's time travel devices to leave Doomsday stranded at the End of Time, where Doomsday met the one force he could not overcome: entropy.