John the Revelator (also known as John of Patmos) is a character in History that is referenced within DEXTER. John is the author of the Book of Revelation and is the supposed owner of an ancient sword known as the Sword of John the Revelator.

Professor James Gellar was a teacher at the University of Tallahassee who taught in religious studies and he had a fascination with the End of Days predicted by the Book of Revelation. In studying an Early A.D. Cult known as the Enesserrette, he learned that they believed to have discovered a Code that could be used in bringing about the End of the World. Gellar felt that he had deciphered the code and recorded various notes on the requirement for how the end of days would occur. As an experiment in curiosity, Gellar performed an experiment with The Lamb using a dead lamb as well as his teacher's assistant Clarissa Porter though got in trouble somewhat with the University he was teaching at. One of Gellar's students however, Travis Marshall, became obsessed with the properties of Gellar's notes and eventually stole the Sword of John to prove that he and Gellar were the Two Witnesses. Unfortunately, Gellar loses his life when Travis stabs him to prove his "inability to die" and Travis carries on alone as the Doomsday Killer.

Using the Book of Revelation as reference, Travis attempts to bring about the end of the world by enacting signs depicted within John's writings. Thus far he has committed five murders as "Signs", and is attempting to bring about two final signs (Wormwood and Lake of Fire) before the End of Days.