Mary Shelley's novel Frankenstein tells the story of Victor Frankenstein and the monster he created. Frankenstein is consumed with the idea of bringing the dead back to life. He is able to create a body from stolen body parts and bring it to life. But when he does he is so frightened by what he has created he runs, leaving the monster behind.
The monster tracks Frankenstein down and in the process kills Frankenstein’s younger brother William. The monster demands that Frankenstein make him a companion. Initially Frankenstein agrees to do so but then changes his mind. The monster is enraged and promises that he will be with Frankenstein on his wedding night.
Frankenstein returns home to marry Elizabeth. It is clear that he is haunted by the creation of the monster, the murders the monster committed and his threat. Elizabeth wants to know what is upsetting Frankenstein but he does not want to frighten her. He promises to tell her what is bothering him after they are married.
He doesn’t get the chance to tell her though. The monster kills Elizabeth on their wedding night.
He says he has a dreadful secret that will "chill your frame with horror, and then, far from being surprised at my misery, you will only wonder that I survive what I have endured." He promises to reveal this secret to her the day after they wed. He will wait until then because there "must be perfect confidence between us." He doesn't feel secure telling her until they are married and have that trust and confidence.
In Mary Shelley's Frankenstein, Victor Frankenstein promises to tell his betrothed, Elizabeth, the reason for his "misery and terror."
Victor wrote to Elizabeth "I have one secret...; when revealed to you, it will chill your frame with horror, and then, far from being surprised at my misery, you will only wonder that I survive what I have endured." (page 233)
Victor also mentions that he will only confide this "tale of misery and terror" after they get married as he wants "perfect confidence" between them. Victor, basically, wants to tell Elizabeth all about the Monster.
Victor is promising to tell Elizabeth about the monster he has created that has been haunting him ever since. He wants to be married to her before he tells her because he wants to ensure that she will not abandon him and shame his name with the truth of his creation. He has already lost so many people near and dear to him because of his monstrosity; he does not want to lose Elizabeth when he reveals his secret creation to her.