Annie injects herself with the poison containing uranium (the only substance that makes the poison work against the monster) that was prepared by Nodin in order to kill the monster. The monster kills Annie and dies itself as soon as it comes in contact with the uranium in the poison.
The story describes Annie using a poison made by Nodin that contains uranium to kill the monster. This poison is the only thing that can harm the creature and causes both Annie and the monster to die when they come into contact with it.