The result of Mary Gray overdosing on the t-Virus contained in her wrist bracelet and mutating into a large bio-organic weapon.
Mary herself had already adapted perfectly well to the t-Virus but due to her delusions of how she saw the world, became angered at what she saw as lesser humans and those with a purpose.
After resisting arrest from the B.S.A.A, she triggered the mutation by overdosing on the virus causing the mutation. Monster Mary grew to an enormous size yielding large claws similar to that of a Super Tyrant. She was however much larger and was able to sustain heavy gunfire attacks.
Despite this, she was eventually destroyed on the rooftop of the university after Chris Redfield fired a rocket into the beast ending the life of Mary Gray.