(Capcom's primary storyline)
The necrotoxin is a toxin designed by the Connections for the elimination of their human bioweapons. Each individual series of B.O.W. has its counterpart necrotoxin that can only be used against. For example, E-Necrotoxin is instructed to be used against E-Type bio-weapons.
During the Dulvey incident, Ethan Winters was able to synthesis the E-Necrotoxin at the Abercrombie Salt Mine from a genetic sample handed to him by his wife, Mia, an agent for the organization. However, when used on Eveline it caused her to mutate into a large mold-based creature before death.