Narcoblixes are a species of Blix and they have the power to enter a trance and control the bodies of those people they have bitten, but only if those people are asleep. Vanessa Santoro is a Narcoblix who bit the other characters in their sleep in an attempt to steal one of the five artifacts (the Sands of Sanctity) and overthrow the preserve. She was thwarted and put into the Quiet Box to keep her from controlling those she had bitten.
In Secrets of the Dragon Sanctuary, Vanessa told Kendra that as her mind was resistant to mind control, she could not control her in her sleep despite having bitten her. This implicates that Narcoblixes use a kind of mind control to control people in their sleep.
Wayne was another Narcoblix introduced in Keys to the Demon Prison. Later in the same book, Vanessa and Bracken have a slight Unicorn versus Blix rivalry because of the many times Unicorns hunt Blixes.