Director Tad Stones has posted on the official Hellboy Animated "production diary" that The Phantom Claw is the title of the third film in the series:
I was going to wait until the production has a green light but the script for the next Hellboy Animated is headed into the climax and I wanted to talk about the specifics of the creative process in something close to "real time." So ladies and gentlemen, boys and girls, let's begin the story of The Phantom Claw.
After Sword of Storms was underway, Mike and I got together to talk about the second movie. We thought we'd cover Hellboy's origin and tie it to activities that were underway in the same location today. What if someone uncovered all the WW2 equipment and the spirit of Rasputin guided them toward recreating the process that brought Hellboy to Earth? I don't think we wrote a full treatment but it was a very long, extended premise and all the key points were in there.
Generally, we were left on our own to create these movies but Lloyd Levin, one of the producers of Guillermo del Toro's Hellboy movie, called up Mike. His point was that we were telling the same story as the movie. "The fans have read the origin in the comics and saw a variation of it in the movie. The animated films should be about expanding the franchise." My knee jerk reaction was that it wasn't the same story at all except for the origin. It was nothing like the movie. But Mike pointed out that a large chunk of it was and that it started the same way as the live action movie. He said we should stick our necks out and do something original.
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