The Dimidius starts to lose their powers before they turn eighteen. Their vampire and human sides are at odds. Once they become eighteen they have the choice to let their vampire side win or turn human. If they walk out into the sunlight on the dawn of their eighteenth birthday their vampire side will die. If they stay in the shadows their vampire side will win and become dominant, suppressing their human side.
The most notable Dimidius is Vladimir Dracula, who was conceived by the human Sally Giles and the vampire Count Dracula. After Vlad's birth, the Count decided to adopt Vlad without Sally's permission. The secret about Vlad's real mother and his real identity were kept secret from him for nearly 18 years.
- "Ahead of you lie two paths, which will you take? One path leads to immortality, and the other, the other... leads to death."
- --Grand High Vampire, Insomnia
In his dream Vlad turned into the sunlight when the Grand High Vampire asked him to finish him and take his rightful place on the throne. In "The Darkest Hour" Vlad did the opposite and chose to be a vampire instead of a human.
- In Latin 'Dimidius' means half or incomplete. It is probably quite derogatory to call a vampire Dimidius.