How do you fix the sentace Elmer slumped down in his seat his worst fear was realized?

To fix the sentence, you can add a conjunction to separate the two independent clauses. For example: "Elmer slumped down in his seat, as his worst fear was realized." Alternatively, you could use a period to create two sentences: "Elmer slumped down in his seat. His worst fear was realized." Either option improves clarity and readability.