Mephisto (real name unknown) was a homeless man and the leader of the underground homeless community called Route 666. He was a tall man with long gray hair. He wore dated 70s-vintage clothes with hippie accessories including a Navajo squash blossom necklace. Mephisto was a Vietnam veteran, an infantryman with the I-Corps of the Seventh Infantry.
Mephisto's territory included several areas under and alongside Central Park. He and his lieutenants ruled and protected the tunnels and kept their people safe from disturbances by the police and the occasional predation of other homeless. Prior to the events of Reliquary, several of his community members had been brutally killed when they ventured too far from the community. Through his information network, Mephisto learned that a new group of homeless, the Wrinklers, had moved into a far-flung corner of the underground known as the "Devil's Attic". It was rumored that the Wrinklers were sub-human monsters and were responsible for the killings.
Mephisto sent a runner to contact journalist Bill Smithback, who had written an article about the murder of Pamela Wisher in the New York Post. The runner met Smithback in a men's room at Penn Station and led him into the tunnels. Smithback refused to go very far underground, but he spoke to Mephisto through a grate in the floor. Mephisto called Smithback "Scriblerian", a reference to the writings of Alexander Pope. He told Smithback about the underground killings and Smithback duly published the article.
Later, Special Agent Aloysius Pendergast and Lieutenant Vincent D'Agosta visited Mephisto in the Route 666 Community. Posing as the leader of another homeless community and his lieutenant, the two men shared a meal of roasted rat ("track rabbit") and cheap wine with Mephisto. Mephisto told them about the Wrinklers and the Devil's Attic, and Pendergast resolved to find out all he could about the fabled area.
Pendergast identified the Devil's Attic as the deep and barely-accessible Astor Tunnels, built in the late 19th century for a private rail network accessible only to New York's wealthiest families. He made several trips down and discovered that the tunnels were indeed the home of the Wrinklers, and they were something other than ordinary humans.
Later, Mephisto and his community were rousted from Route 666 by the New York Police Department, in preparation for flooding the entire area in an attempt to drown the Wrinklers. Pendergast dragooned Mephisto to lead him and D'Agosta down into the Astor Tunnels to detonate explosives and prevent the Wrinklers from escaping. Mephisto agreed, but only on the condition that no matter the outcome of the evening, Pendergast would personally intervene for the future safety of the Route 666 community.
Mephisto, Pendergast and D'Agosta descended into the tunnels, and were later joined by Dr. Margo Green and Bill Smithback. They found the Wrinklers in the Astor Tunnels and were attacked and captured. Fortunately, the arrival of a police diver with heavy weapons created a moment of confusion. Mephisto detonated a Claymore antipersonnel which he had stolen from an FBI weapons locker. The explosion killed Mephisto and dozens of Wrinklers and allowed the the rest of the group to to escape and detonate the roof of the tunnels, trapping the Wrinklers inside.
Just prior to his death, Mephisto gave Margo Green an envelope and asked her to deliver it for him. The envelope later became waterlogged and too smudged to read, but Margo opened it and and found it contained a photograph of a young boy in a train engineer's cap.