He was the Dungeon Master in their Dungeons & Dragons games. John Romero loved Carmack's campaign world, but John Romero destroyed it, Opening up a gateway to hell, and then immediately regretted it (but they got the idea for Doom from it), since it was a long running campaign in a world completely designed by John Carmack. I think Heretic/Hexen and Quake also borrowed heavily from their D&D games. Masters of Doom is an awesome book.
