Synonyms:
* Preamble: A preliminary statement, especially an introduction to a formal document or speech.
* Preface: An introductory part of a book.
* Introduction: A part that comes before the main body of a piece of writing, music, etc.
* Prolegomenon: An introductory part of a book, especially a long one.
* Overture: An introductory movement or section in a musical work.
* Precursor: Something that comes before something else and often leads to it.
* Harbinger: A person or thing that foreshadows or announces the coming of something else.
Antonyms:
* Epilogue: A concluding part, especially of a literary work.
* Postscript: A short addition at the end of a letter.
* Coda: A concluding section or part of a musical work.
* Finale: The final part of a performance or event.
* Conclusion: The end or finish of an event or process.
The best synonym or antonym to use will depend on the specific context in which you are using the word "prelude."