VampireInterior

 It goes down the list of the organs, detailing the differences in vampires that have been successfully autopsied. The largest change is the digestive system, which alters itself quite significantly to absorb the life force of blood. The side effects of this change include a slower metabolism, and a much stronger liver. Second is the skin, which becomes much more elastic. It also becomes able to harden itself almost on command among older vampires. Third is the heart, though its changes vary from vampire to vampire. Some stop beating entirely, some magical force keeping blood flowing. Others beat rapidly until the body is injured, then slowing itself to lessen the severity of possible blood loss. Finally is the bone structure. Quite a bit of it hardens and increases in density, allowing for more force from the muscles to be applied to them without breaking. This contributes to the increased strength. All other organs and functions are enhanced, though the level of enhancement increases with age.

Its an entire book on Vampiric transformations. The author notes that this only applies to vampires of over 500 years old, and they need a fresh supply of blood to do these acts. The transformations all share a common theme of either bat or wolf based. The most common transformation is turning into a bat. Notable other transformations are monstrous werebats, shadow wolves, and a werewolf like form. However, there is one that has been reported but never confirmed. During the last war against necromancers and the undead, a group of Paladins of Aurora reported fighting a Vampire that become some large almost demon like creature. The reports from the ones that maintained sanity and lived through the battle describe the creature as a fifteen foot tall creature with pitch black skin and large bat wings. No eyes, but it had a massive shark like mouth on his head. The shape was vaguely humanoid, aside from the protracted snout from the head. Of the twenty paladins that fought the creature, only three survived the encounter with their mind and bodies intact.