- See also: James Marcus
|Created via:||Progenitor virus, primary-infection|
The Queen Leech was one of the most uniquely grotesque monsters to emerge from experimentation with the Progenitor 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 merged with his DNA to create a new being, eventually succeeding after ten years.
The reborn Marcus, consumed with hatred, vowed a terrible revenge on Lord Spencer and Umbrella for their attempt on his life that cost him his humanity. To this end, Marcus 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.
All the inhabitants of the train were killed and transformed into zombies due to the high concentrations of the t-Virus Marcus unleashed. Marcus effortlessly shirked off U.S.S. commando attempts to clean up the source of the biohazard. In control of the situation, Marcus displayed a video transmission across the training facility, where Albert Wesker and William Birkin remained in hiding from the zombie population, threatening his murderers that they would pay for their act of betrayal.
When S.T.A.R.S. Bravo team officer Rebecca Chambers and Ex-USMC Second Lieutenant Billy Coen were lost in the forests of Arklay, hunted by B.O.W.s, they soon encountered several creatures that mimicked Marcus during their search of the train and training facility. Eventually, Rebecca and Billy confronted Marcus himself, in an underground location under the facility.
After revealing details regarding his mortal demise, Marcus uncontrollably mutated into a leech-like monster after the queen began to control his body. Although the new creature was impervious to small-arms fire, Rebecca and Billy managed to defeat it by exposing it to sunlight; its one weakness, and Billy finished off what remained of James Marcus with a single bullet.
Though Marcus did not live to see it, the events he 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 gained his vengeance on Spencer and Umbrella after all. However, it also resulted in Wesker's rise to power.