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The Worldkillers were a team of five genetically engineered aliens that were part of a secret research initiative made by the Kryptonians. Their goal was to collect various embryos from various species across the universe after which experiments were conducted to study them, test them and change these female specimens. The code name given to this project was Worldkillers that was a term taken from Kryptonian mythology. These experiments were conducted not on Krypton but rather a lone laboratory that floated in space and away from prying eyes. A total of five specimens were created but at some point the experiment was abandoned though the subjects remained within the laboratory where they fully formed. The fifth specimen had long since disappeared whilst the remaining Worldkillers awoke and attempted to learn their origins. From the broken datacores, they discovered fragmented pieces of their origin but were unaware of the reason of their existence. All that they were left with was a hunger to conquer and destroy along with the need to find the homeworlds of their respective species origin.
Using their ship, the four Worldkillers departed and discovered that Krypton had been destroyed in a cataclysm. Thus, this race of conquering warriors that traced their origins to Krypton sought out further flues to their mysterious origins. Each of the superhuman Worldkillers were incredible powerful and had the ability to destroy an entire city or world. This was described as being a time consuming and rarely thrilling experience. Furthermore, Reign claimed that the Worldkillers were expert warriors and that only another Worldkiller could kill one amongst their number. In addition, she claimed that since they were harvested that the Worldkillers were still evolving to their true potential. Only after their awakening did they come to realize their power.
Reign of the Worldkillers later managed to encounter the dead remains of Argo City that had somehow been ripped from Krypton's surface. This city was orbiting a blue sun and no Kryptonians populated the settlement. However, from its datacores she managed to uncover a new trail to the planet Earth and led Reign to believe that some Kryptonian secret laid on the world. During this time, she encountered Kara Zor-El who was attempting to learn of her homeworlds fate leading to a clash between the two. Reign later arrived on Earth and began attacking a Human city only to encounter Kara Zor-El who had managed to survive their previous encounter. Impressed by her abilities, Reign offered Kara membership into their group as she believed that the two had a lot in common. When Supergirl refused, Reign summoned the other three Worldkillers and placed a force field similar to the one that encased Argo City to separate the Human settlement from the world. The Worldkillers battled and though she put up a good fight she was quickly being overwhelmed. After rejecting another offer of membership into the Worldkillers, Kara recalled Reign saying that only one of their number could slay each other. Thus, she used Perrilus's hands to poison Deimax leading to Reign ceasing hostilities as she could not afford to lose a single Worldkiller. She departed the battle but left a cryptic warning to Kara Zor-El that there was a fifth Worldkiller that was released and was somewhere in the cosmos.
- The Worldkillers were first mentioned by Reign in Supergirl (Volume 6) #5 though the group only featured at the end of Supergirl (Volume 6) #6 whilst their origin story was revealed in Supergirl (Volume 6) #7.
- The concept of a genetically engineered creature designed to destroy worlds from Krypton is similar to the previous origin story of Doomsday.