A cytotoxin is any compound or molecule that has a toxic effect on cells. In the world of poisons, the cytotoxins that are most interesting are those contained in the venom of snakes.
Many of the most deadly snakes in the world have cytotoxic venom: Cobras and Adders
Cytotoxic venom contains several digestive enzymes and molecules known as “spreading factors” which cause both local and systemic injury.
Locally the venom causes pain, swelling, bleeding and blistering around the bite mark with eventual necrosis (rotting) of the tissue in the area of the bite. Systemically (throughout the body) anti-clotting proteins in the venom can cause system wide bleeding and organ damage.