The Inhumans are a 7,000-year-old group of early humans that were experimented on and given powers by the Kree. The Kree previously appeared in the Marvel's Cinematic Universe, in Guardians of the Galaxy (2014) and Agents of S.H.I.E.L.D. (2013).
While Black Bolt could not speak without causing danger, it is irresponsible to have only one person (Medusa) who could translate a king's sign language to his people.
$1,135,440 3 September 2017