- See also: James Marcus
The Queen Leech was one of the most uniquely grotesque monsters to emerge from experimentation with the t-Virus. The 'Queen Leech' was one of Dr. Marcus' pet leeches used in his experiments that began to grow a collective intelligence. After Marcus' death on the orders of Oswell E. Spencer, his 'queen' leech absorbed his hippocampus into its being, leading to it believing itself to be Marcus himself. Like the smaller Leeches it too was mimetic and could assume the appearance of Dr. Marcus, although to a far greater degree.
The Queen Leech was created in 1988 as part of Dr. Marcus' research into the t-Virus, the doctor having shifted Leech research from the Progenitor Virus. He was assassinated by an Umbrella security team during his experimentation, and the two were dumped into the underground river below his lab. The leech entered Marcus' body through his mouth and began eating his body from the inside. Eventually it worked its way to the hippocampus, where it absorbed his long-term memory. This began a process in which the remarkably-intelligent creature began to believe itself to be Marcus himself, somehow revived thanks to the Queen Leech. A religious man, the Queen Leech interpreted his spirituality as an explanation, comparing itself with Jesus Christ's resurrection in the Christian tradition.
From Marcus' memories, the Queen Leech shaped its body to that of Marcus' teenage self. Remembering Marcus' demise, it now sought to avenge the doctor's death. Now ten years on from the death in 1998, the Queen Leech unleashed the t-Virus upon the Arklay Laboratory and the Research Center, and then attacked the Ecliptic Express, a personal company train devoted to luxuriously transporting Umbrella employees. The Leech then ran quickly back to the Research Center, where it taunted Albert Wesker and William Birkin through a camera feed in the broadcasting room, threatening his murderers that they would pay for their act of betrayal.
During the night, the Leech showed a degree of interest in S.T.A.R.S. officer Rebecca Chambers and ex-Marine Second Lieutenant Billy Coen, who were slowly uncovering Marcus' human experiments and the conspiracy to assassinate him. Eventually it chose to confront them in the nearby water treatment plant.
After revealing details regarding "his" mortal demise, the Leech began to transform into its 'true' form. Although it was found to be impervious to small-arms fire, Rebecca and Billy managed to defeat it by exposing it to sunlight - its one weakness. With a single bullet, Billy was able to seriously injure the Queen Leech, resulting in it dropping down a chasm into an on-going explosion.
Though the Queen did not live to see it, the events it had set in motion eventually led to Raccoon City's destruction and the deaths of most of its population, along with the downfall of the Umbrella Corporation, which turned Spencer into one of the world's most wanted fugitives. This ensured Marcus had been avenged.
The Queen Leech's transformation in its last moments was eventually found out by Wesker some time after the events at the Umbrella Executive Training Center, which led to his theorisation of a connection between t-Virus mutations and the psychology of the infected.
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