… developing a fantasy game with flying fairies.

In an interview with PC Gamer, Doug, Erik Johnson, and Gabe Newell:

Doug Lombardi: There were a few failed starts to build Left 4 Dead.
Gabe Newell: Well, there was the flying fairy game. Is that the one you were referring to?
Erik Johnson: That was just a different game that, when we stabbed it… (everyone laughs)
Doug Lombardi: … It turned into Left 4 Dead!

Erik Johnson says it was a weird prototype game that had spells: “based on movement and mouse gestures. It was so bad.”

Gabe then says:

“It was so bad, you wanted to ask yourself: ‘How could we make a game that was this bad? And how should we make a game?’ And we said we should focus on what we do really well, so why are we doing this game which was kind of a… it wasn’t really an RPG… it was this action fantasy sort of role playing game that had no story. And then we said ‘OK, that’s so horribly wrong. What we should focus in on is AI and playing in co-op, and that’s the interesting opportunity.’ That was where Left 4 Dead came from.”

Check PC Gamer’s original article here: http://www.pcgamer.com/2010/08/26/valve-were-making-a-fairy-rpg-before-left-4-dead/