An aboleth is an extremely powerful aberration, usually residing deep in the Underdark. Next to nothing is known about them, despite the fact that they are incredibly ancient and long-lived.
During their travels through the Underdark, Vox Machina came to an underground lake. They were warned by their new ally, an illithid named Clarota, that an ancient evil lurked beneath the waters. When pressed for more information, he named the evil an aboleth, and said they would be much better off if they avoided it altogether.