The Inhumans are an isolated group of superhumans descended from primitive humans who in ancient times were experimented on by the Kree. Not to be confused with the Eternals who are an isolated group of superhumans descended from primitive humans who were experimented on in ancient times by the Celestials. The Inhumans, like the Eternals, are a group of Marvel Comics characters that I could never develop much of an interest in. Not that there have not been some good Inhuman stories over the years. Paul Jenkins and Jae Lee's Inhumans miniseries from back in 2000 comes to mind. Not much else though.
For a while there has been a rumor that Marvel Studios is planning a Inhumans movie franchise. During their bankruptcy in the 1996 Marvel sold the film rights to the X-Men to Fox. Now Marvel wants a property they can use in place of Mutants in their expanding film universe and the Inhumans could be it.
In Inhumanity the floating city of the Inhumans has been destroyed and the terragin mists that give them their powers have been released into the atmosphere, which is an idea from Alex Ross' Earth X miniseries from back in 1999. Please note, these events do not happen in this graphic novel. It happened it a previous book and Inhumanity tells the story of the aftermath. Thousands of people all over the world are transformed into Inhumans, sometimes with grotesque forms and strange powers, much like mutants do when they first mutate. The new Inhumans are feared by the public and hunted by governments and corporations that wish to exploit them, much like mutants.
A better name for this series would be New Mutants, but Marvel has already used that.