NET
18 I testify to everyone who hears the words of the prophecy contained in this book: If anyone adds to them, God will add to him the plagues described in this book.
Darby
18 *I* testify to every one who hears the words of the prophecy of this book, If any one shall add to these things, God shall add to him the plagues which are written in this book.
KJV
18 For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book: