Rise of the Flayers
Aberrations are creatures that are so different from others or so alien that they are grouped together. Mind Flayers, Beholders, and Neogi are a few examples.
There are very few known Aberrations in this world. Those who read ancient texts know of them and where they like to live, but not how to find then. There are also many ancient texts on how to fight them. None seem to agree, however, so they are not of much help.