The Queen Leech was a mutant leech created through experimentation on annelids with the t-Virus. Created as part of Dr. James Marcus' ongoing leech study, it was his final creation before his death. In the time after, it consumed his hippocampus and gained his memories. Believing itself subsequently to be Marcus reborn, it sought revenge for his death.
The Queen Leech was created in 1988 in a laboratory adjacent to Umbrella's executive training school in the Arklay Mountains. When Dr. Marcus was assassinated by the Umbrella Security Service, Queen Leech was dumped in a nearby treatment plant with his corpse. Seeing his body as a food source, Queen Leech began consuming it, and gained his memories after consuming the brain. It believed itself to be Marcus himself, reborn by divine intervention.
Over the next ten years, the Leech developed the ability to change its body shape into that of Marcus'. In May 1998, the Queen Leech finally acted on its plan to avenge Marcus' death. Over the next few weeks it manipulated an accident at the Arklay Laboratory, leading to the entire facility staff being infected with the Epsilon strain, while their B.O.W.s escaped into the forest. Umbrella's investigation teams sent to evaluate the training centre and laboratory for recommission were attacked and infected by leeches, as were the entire compliment of a passenger train transporting Umbrella staff through the Arklays. Queen Leech's goal was to create a devastating viral outbreak that would ultimately spread across the world. In keeping with Marcus' Christianity, Queen Leech saw itself as Jesus Chris, and judged the entire planet to be wicked and be condemned to Hell.
These attacks got the attention of Drs. Albert Wesker and William Birkin, who were in charge of the training centre operation. The two quickly recognised Queen Leech as a serious threat to the company and made preparations to destroy the facility. In the meantime, Queen Leech's attention was focused on S.T.A.R.S. member Rebecca Chambers and former Marine Second Lieutenant Billy Coen, who found their way in the training centre when the train crashed into the underground station beneath it. Queen Leech eventually saw no other way than directly confronting the two after they survived repeated attacks from Marcus' B.O.W.s. It abandoned its human form and transformed into its natural state, a large green monster. The fighting went awry when it accidentally revealed to Billy its sensitivity to sunlight. Distracted by the opening of a shutter, Billy fired at Queen Leech with a high power magnum, and its body fell down an elevator shaft where it was consumed by fire as Dr. Birkin set off the self-destruct.
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.