"In the End" and "At the End"

In the end  means finally, after all.

• In the end they decided to make a deal.
• In the end, it turned out that it was Carlos who shot Alex.

By at the end we mean that something is adjacent to an extremity.

• Rowling left the question of their relationship open at the end of the book.
• At the end of the town, there is as a beautiful villa, called Mistery Villa.