Fantasy series - Kids playing D&D get drawn into the game one becomes a vampire

[Larry]

The one specific scene I remember is when they end up in a vampire castle with no way out, so they place themselves into a bag of holding and throw them selves off the cliff edge and one of them is a druid and turns into a bird to fly down and open the bag to let them out.
One of the kids had never played D&D before and got pulled into the game and got turned into a vampire and spent a lot of time in the bag of holding