Aberration

From the Far Realm, Aberrations are extremely dangerous creatures. They are parasites of all the planar spaces. The portals between the planes work like pores to the center which is the material plane. They are not really known about in the material plane since that is the deepest center-most plane of all the planes and, for the most part, has too thick of a layer to penetrate. As such, the people of the material plane are safe and ignorant to the horrors that all the other planes must deal with.
Aberrations have a desire to survive and procreate on a molecular level that most other creatures could not achieve. This drive for survival means that they can change their form to best prey on the plane that they currently reside in. Some of these planar aberrations are not harmful, often due to them being driven mad by their environment. However, for the most part, aberrations from any plane are dangerous creatures. Some examples of these include the Cloaker from the Plane of Air and the Flumphs from the Feywild.
There is an order of humanoids known as the Aasimar that understand the danger that the aberrations possess and are trained from birth in how to kill them.
For more information on the planes based on the research that Kwalish has done, Click Here.
At this point, you know that the Nowhere King is also an Aberration known as an Aboleth.