Stross, Charles. Laundry files novel

"In 1984, Derek Reilly was just another spotty teenage dungeon master growing up in middle England. But then a secret government agency tasked with suppressing magical intrusions received a tip-off - and one midnight raid later, his life was turned upside down by the Satanic D&D Panic. Decades later Derek, now middle-aged and institutionalized, is a long-term inmate at Camp Sunshine, a center for deprogramming captured Elder God cultists. He's considered safe enough to edit the camp newsletter, and he even has postal privileges - which he uses to run a play-by-mail game. After 25 years, Derek finally has reason to escape: a nearby D&D convention. While Derek's D&D games were full of fictional elder gods and world-ending threats, a LARP game at the con is a dread ritual designed to summon a great evil into our world, and it's up to Derek and his players to stop them"-- Amazon.com