The Inhumans are a race of superpowered individuals first introduced in Fantastic Four #45. Inhumans have their powers awoken through an alien substance known as the Terrigen Mist. While the Mist gives powers to people who are already Inhuman, the substance has no effect on normal humans. The only problem is that mutants are actually negatively effected, leaving them poisoned and sterile.
The problem then is that the Inhumans wish to spread the mist to awaken more of their kind, while mutants would be left to die if that were to happen. The Inhuman and Mutant factions are led by the Inhuman royal family and the X-Men respectively. While the two have joined forces in the past, it seems that the two-week survival period of the mutants forces them into trying to stop the Inhumans from advancing their plans.
WHO WILL WIN?!
via Marvel Reveals Why the X-Men Will Fight the Inhumans | Inverse