Nightthief

Description
A green liquid created to allow non-Perfect Travelers to remain in control of their vesel. This then means they do not require constant reminders and their original sleves of that dimension can't regain unknown control. Even with the use of Nighttief, vesels have so far been recorded to still not have memory of the travelers using them (copared to Perfect Travelers where the vesel remebers everything).

History and Uses Throughout the Books
Nightthief has been shown to deteriorate the vessel (as displayed in the Burkleverse Theo after being taken over by the Triadverse Theo).

There is still no known complete cure. The Home Office created a somewhat cure that allows the person's immune system to rebuild itself and fight off the Nighttief.