Why is Grendel missing a toe?

Part of Grendel's undertakings have taken him into climates that are freezing cold. As such, Grendel has experienced frostbite. Frostbite mainly effects the extremities such as fingers and toes by causing the arteries/veins/capillaries to constrict and hinder the flow of blood and sometimes stop it. When blood flow stops, this is called gangrene, which results in the loss of the extremity. Grendel has lost a toe due to the effects of frostbite.