The word that matches your definition is "inevitably."
Inevitable means that something is going to happen one way or another, and there is nothing you can do to stop it or prevent it.
The word is used in the story when Rainsford decides that there is no way he can go straight when running away.
He saw that straight flight was futile; inevitably it would bring him face to face with the sea.
When something is inevitable, it is bound to happen. This means that if Rainsford keeps going, he will run to the sea. Once at the sea, he is not going to be able to get away without swimming, and he cannot swim to safety because there are so ships out there.