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The format was similar to the Tower of Gygax events that have been run at GenCon the last couple of years. Players could sit down, grab a pre-gen character sheet, and join in the fight. When they died, they stood up and someone else sat down to play for a while. Mike also kept track of how long the players had been in the game; when no one died and there was a player standing around waiting to jump in, he'd boot the longest-living player. It was all in good fun, and the players cheerfully vacated their seats to let the newbies sit down.
As for me, I live 4 blocks down the street from shop, so I played two separate sessions: one before noon, when ol' Chgowiz was bright-eyed and bushy-tailed, and another toward the end of the evening, when he had the thousand-yard stare going.
The game was Holmes D&D, which had a few interesting quirks that I learned about (clerics have no spells at 1st level, oil does crazy damage, adventurers can walk three abreast down a dungeon hallway).
Throughout the day, there was remarkable continuity in terms of player progress. When the mapping player's character died, he'd pass along the map (which had probably been started 2 or three players ago) to the next likely mapping player, so he could take up the pencil for an hour or so. Likewise with treasure and magic items looted from the Caves of Chaos. We just passed 'em down the line. A healing potion filched at noon might finally be used around 5:30 p.m. by a new player.
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Each player who survived an hour got a free d20 and a d6, compliments of the good folks at Chicagoland Games. Mike also had some very nice printed copies of Matt Finch's Quick Primer for Old School Gaming, which he gave out to players as well. (This was probably the ideal context to distribute this most excellent treatise, following on the heels of a glorious one-hour old school dungeon romp.) In the end, Mike estimated we probably got through 20 percent of the module. I'd guess 15 to 20 players rotated in and out of the game throughout the day. Some came back three, four, even five times to play new PCs.
The best part came during the final hour of the game. Two players who had never played D&D—or tabletop roleplaying in general—stopped by the table. I'm guessing they were a boyfriend/girlfriend combo. They said their only experience with this type of geekery so far had been playing World of Warcraft. So of course, we happily booted some players to make room for them. Mike explained their character sheets, we pointed out what was on their equipment lists, and they plunged into the fray. There was much laughter and gnashing of teeth for another hour or so, and then Chgowiz called the game around 9 p.m. Applause broke out at the table—he'd been at it for 11 straight hours.
Afterward, I asked the two newbies how they heard about this D&D marathon. Stunningly, they had simply googled "D&D chicago" a few hours before—having never played, only heard about the game from a friend—found the game store's website at the top of the results page, found this event on the store's online calendar, hopped in their car, and drove out to play. Their whole day of gaming coalesced in less than six hours. After the game, they inquired about the weekly D&D 4e game that takes place each Monday at the store, and they left with plans to return next week. It's an amazing world we live in.