The Alien King was a genetically-engineered Xenomorph created by Professor Ernst Kleist through the use of genetic engineering in an attempt to create a weapon to rival the Queen caste, and thus help rid Earth of it's Xenomorph infestation.
Professor Kleist was assigned by the ZCT to oversee Project Chimera being conducted on Charon Base, which intended to breed a race of domesticated Xenomorphs through the use of genetic-engineering. Though Professor Kleist was able to create several domesticated Drones, he was unable to breed any Queens as they all would automatically die upon birth. Frustrated by his failed attempts to breed a Queen, Kleist decided to breed a Xenomorph powerful enough to usurp a Queen. After uncovering the true identity of corporate spy, John Kray, Kleist had him restrained and impregnated with his genetically engineered Facehugger. Rather then let the Chestburster violently erupt from Kray, Kleist had it safely extracted from the former's body. It eventually matured into a giant behemoth, which Kleist dubbed the "Alien King". Kleist had the restrained Kray meet his "son", though it's reaction was not what the Professor had anticipated. Rather that kill Kray, the King choose to examine him closely at first and finally ignored him completely. Kleist had determined that he had finally created a perfectly obedient Xenomorph, deeming it a perfect evolutionary successor to the Xenomorphs and hoped to have his King challenge Charon's resident Alien Queen.
Professor Kleist introduced the Alien King to the base's captive Xenomorph hive to see it's reaction, only for it to backfire completely as the King began to massacre the hive's entire population with little effort. Rather than see it as a failure, Kleist changed his view of the King as the perfect biological weapon for the ZCT to use. After hearing the Queen's cries, the Alien King broke out of the testing area and proceed to locate the Queen. Its recklessness, however allowing the remaining Xenomorph Drones to pour into the human sector. The King continues to run rampant through the entire Charon Base, killing both Xenomorphs and Humans alike. After killing off the Queen's Royal Guards, the King finally confronted the Queen herself, preparing to duel her to the death. While the initial fight was in the King's favor, the Queen's superior speed and intelligence enabled her to easily dodge the King's brutish attacks while she slowly wore it down. With the King weakened, the Queen finally delivers the final blow, killing it. Unable to accept defeat, Professor Kleist used a sound cannon on the Queen until it went critical, destroying the Charon Base.