I believe the initial antagonist of this Inhuman series should be another Black Bolt cousin: the exiled Unspoken. Before Black Bolt, the king of the Inhumans was a character with the ability of mass manipulation and the manifestation of energy constructs. Basically, he’s the embodiment of Terragenesis itself and the perfect Apocalypse surrogate for the Inhumans.
For this role you need German-Egyptian actor Mido Hamada (Terra Nova, American Sniper, Homeland). He’s an imposing guy, and has the TV-acting experience for a great two-episode, special appearance. After playing that villainous role , he can be exiled back to London, which is where he resides. I know him better from Terra Nova, and he can be a scary guy. Add the appropriate costume and make-up and I’m sure the Inhuman’s boogey man would be ready to go.