Dungeons & Dragons turns forty this year. The game, which I played in my youth, is entering middle age just a few years behind me. My interest in—or, I should say, my obsession with—D. & D. coincided with the height of its popularity, in the nineteen-eighties. D. & D. was more than just a fad or a hobby. It was a subcultural sensation that popularized the idea of role playing and ushered in a seminal change in the way games were created and enjoyed. Instead of pieces or figurines, there were characters—avatars—who the players inhabited; instead of a board or a terrain table, there was a fictional world that existed in the shared imaginations of those who were playing; and instead of winning and losing, there was, as in life, a sequence of events and adventures that lasted until your character died. These concepts are now commonplace in our online lives and our recreational activities, but four decades ago they were revolutionary, and a key part of D. & D.’s addictive quality. By 1981, more than three million people were playing Dungeons & Dragons. It soon joined “The Lord of the Rings” and “Star Wars” in a kind of high-nerd trinity—one that, with “The Matrix,” “Harry Potter,” and “The Hunger Games,” has long since entered the mainstream pantheon.