Mana is in all the races in Crepuscule and can be used on rare occasion by humans with the help of an ancient book based solely on teaching humans formulas to manipulate mana. Mana allows them to do magic and comes from the human world - or the outer world as it's being referred to by those who know of it inside of Crepuscule. After Crepuscule - the inner world - was created, the mana began being stolen and taken through the 'forest' to keep Crepuscule alive because it couldn't naturally form on its own inside of the new world. As the mana is constantly being taken from the outer world, the vampires there are slowly dying out on the outside. There are other side effects as well, as Tepes theorized vampires in the outer world were turning into something similar to humans; one example of this is that Priscilla, a pureblood werewolf on the outside, went into labor before giving birth like a human which vampires normally never do. According to Gilles, mana is always being produced like air, as he said "as long as the outside world exists, Crepuscule will exist."
Hundreds of years before Crepuscule was created when humans and vampires lived in the human world both races posses mana then humans started a war with the vampires for unknown reasons killing many vampires in order to stop the war the vampires made a contract taking away the humans mana and every future human. however a very rare few humans found a loophole and kept there mana when they gave birth there children were not humans but something else. There eyes were red like the vampires and they were born with mana and abilities,but it was there bodies that were not human they were akin to the new world as in order for them to live mana was like air to them they absorbed it unconsciously this allowing them to use magic.