It is our deadly enemy. It is what Jesus came to save us from. It is called sin.
Romans 5:12-14 Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned: (For until the law sin was in the world: but sin is not imputed when there is no law. Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam’s transgression, who is the figure of him that was to come.
Teaching by Bro. Brent Evans during Sunday School on the 22nd of June 2014.