WHEN RICHARD GARFIELD DECIDED to propose to his girlfriend 15 months ago, he challenged her to a round of Magic: the Gathering, a card game he created. She won a few hands; then he drew a tailor-made card showing him as a gallant knight offering marriage. "Both players win," it read, "and the decks are mixed as one." His gamble paid off. Says Lily Wu, who wed Garfield last June: "It was the most romantic way anyone could have done it."

Such is the power of love—and Magic, Garfield's collectible trading-card game that draws players into mythical realms of battle and has won legions of fans nationwide. Since the game was introduced 18 months ago, 1 billion cards have been printed, most of them snatched up by teenage boys, sending sales to $50 million last year. On coffee tables and at national tournaments, players take on the roles of wizards and try to kill opponents with the aid of spells and powerful creatures in rounds that can take several hours—or just minutes. Such is the frenzy for the cards displaying fantastic characters like Royal Assassin and Goblin King (costing $7.95 for 60-card starter sets, $2.45 for 12-card booster packs) that fanatics have camped out overnight at stores to lay their hands on new stock. "People get addicted to the game," says Garfield, 31. "It's like a brain virus."

Quite a triumph for an unkempt, mild-mannered fellow who thought he would end up an absentminded professor. A fan of Dungeons and Dragons while growing up in Eugene and Portland, Ore., Garfield majored in math at the University of Pennsylvania. Pursuing his Ph.D. there in 1991, he pitched Magic—which he designed to be portable, fast and inexpensive—to Wizards of the Coast, a game manufacturer outside Seattle. Magic made its debut at a games convention two years later and was an immediate hit.

Garfield, who left a $30,000 job at Whitman College in Walla Walla, Wash., last year, now works full-time at Wizards, where he oversees research for board games. He won't disclose his salary, but he does own 25 percent of the company and has just bought a four-bedroom home 10 minutes from the office with Lily, 31, a Wizards researcher. "It'll be fun to set up house," says Garfield, "so that people can come over and play."