18I warn everyone who hears the words of the prophecy of this book: If anyone adds anything to them, God will add to him the plagues described in this book.
John is basically giving out a threat in order to reassure the readers that this book (Revelation) will stay intact and uncorrupted…
The integrity of the book is preserved by a solemn warning of the danger of losing a part in the tree of life. (John Darby’s Synopsis of the New Testament, Commentary on Revelation Chapter 22, Source)
The supplication of John (which is the third part of the confirmation) joined with a curse of abhorrence, to preserve the truth of this book entire and uncorrupted in two verses. (The 1599 Geneva Study Bible, Commentary on Revelation 22:18, Source)
However, we see that there was a corrupted verse inserted into the book of Revelation before. That verse is Revelation 1:11. The verse is found in the King James Version of the Bible…
 Saying, I am Alpha and Omega, the first and the last: and, What thou seest, write in a book, and send it unto the seven churches which are in Asia; unto Ephesus, and unto Smyrna, and unto Pergamos, and unto Thyatira, and unto Sardis, and unto Philadelphia, and unto Laodicea. (KJV)
However, when one reads the NIV Bible…
which said: “Write on a scroll what you see and send it to the seven churches: to Ephesus, Smyrna, Pergamum, Thyatira, Sardis, Philadelphia and Laodicea.” (NIV)
We can clearly see that the phrase, ” Saying, I am Alpha and Omega, the first and the last” is an interpolated verse.
This wording at the beginning of the KJV‘s version of Rev. 1:11 is not found in virtually any ancient texts, nor is it mentioned, even as a footnote, in any modern translation or in Bruce Metzger’s definitive A Textual Commentary on the Greek New Testament, Second Edition (New York: United Bible Societies, 1994, phone: 800-322-4253).