The Mark (Left Behind #8) by Tim LaHaye & Jerry B. Jenkins

His Excellency Global community Potentate Nicolae Carpathia is back, this time as Satan. Resurrected and indwelt by the devil himself, it's no more Mr. Nice Guy as the beast tightens his grip as ruler of the world. Ensconced in a new safe house, the Trib Force suffers tragic loss at the dawn of the second half of the seven-year tribulation, the Great Tribulation. Meanwhile, Carpathia plans his overt attack on the so-called Judah-ites, while the Trib Force gears up for its most ambitious countermeasures ever. And the battle is launched between the forces of good and evil.