What is a half vampire half werewolf?  They are called werewolf-vampire half breeds.  There are two ways this could happen.  The first way would be if a werewolf is bitten by a vampire.  This is of course extremely rare – but conceivably this could happen. It does not work in reverse however – if a werewolf bites a vampire you do not get a half-breed.  The second instance in which you’d get a werewolf-vampire half breed would be if a werewolf and vampire have a baby.  This could happen, but it is much much less likely to happen – and I am not aware of any cases.  The problem is that werewolves and vampires do not get along, so it is highly unlikely that they would have children together. The only possible way would be if the werewolf would be able to trick the vampire into it while the werewolf is in human form.  This would not happen though because werewolves and vampires have been at war for eons.

If one of the two cases above does happen,  what you end up with is a creature that is a werewolf for all practical purposes – lives as human and transforms during the full moon.  This creature however  does not have a soul and it also lives forever.  The half-breed werewolf thirsts for blood only – not just to attack, but to eat, and it comes out only in the nighttime.  The werewolf-vampire half breed is much much more dangerous than a pure werewolf or pure vampire.