Vanished is a hidden achievement/trophy in Dishonored.
How to Acquire[]
Corvo Attano must escape Coldridge Prison during the first mission and navigate the sewers without being detected. The easiest way to acquire this achievement is to find the alternative paths around large groups of enemies, and to neutralize those who are by themselves.
An example of an alternative path can be seen just prior to exiting the sewer, where there are a few City Watch guards searching for Corvo. The alternative path can be seen prior to entering the area with the guards - piping that leads to a level above them - and Corvo can navigate the area safely, without being seen. Or, for another way, Corvo can swim through the water and past the guards.