Marvel’s Inhumans has already revealed one important connection to fellow ABC series Marvel’s Agents of SHIELD, which introduced the Inhumans to the Marvel Cinematic Universe. It turns out that the Inhuman Royal Family at the center of Marvel’s Inhumans, who live in the secret city of Attilan on Earth’s moon, have been keeping an eye on the emergence of new Inhumans on Earth. Black Bolt, the Inhuman king, even secretly sent Triton to contact some of these new Inhumans and bring them to Attilan,
However, it's possible that there is another connection between the history of the Inhuman Royal Family and Marvel’s Agents of SHIELD that has only been hinted at so far.
In the Marvel Universe, little is known about the parents of Medusa, Black Bolt’s wife and queen, and Crystal, her sister. Marvel’s Inhumans has seized on that lack of history as an opportunity to insert its own original backstory.
Medusa and Crystals’ parents in the Marvel Cinematic Universe were rebels who tried to overthrow the monarchy and the caste system that structures life on Attilan back when Black Bolt and Maximus's father still sat on the throne, at least according to what Medusa remembers the king and queen telling her after the incident took place and what Maximus has been telling Crystal in order to make her sympathetic to his own rebellion. Medusa and Crystal’s parents were exiled for their treason. But exiled to where exactly?
One potential reading of this is that “exiled” is actually code for “kicked them out of the city and into the dark side of the moon to die,” though there seems to be little reason for the king and queen to sugar coat how they’ve chosen to punish usurpers, especially since exile makes them as good as dead to their children. Another possibility is that the rebels were exiled to the mines under Attilan where Inhumans without useful powers are sent, but sending dissenters to mingle with those most likely to be sympathetic to their cause seems like it would be a poor decision on the part of Attilan’s rulers.
It seems more likely that Medusa and Crystal's parents and their allies were exiled to Earth. On first consideration, sending a small, rebellious army to live among humans on Earth seems like a terrible idea if you’re trying to keep your secret society a secret.
However, it should be noted that, at least according to Marvel canon, even Inhumans without notable superpowers can live for at least 150 years, meaning we have no clear idea of when this rebellion took place compared to Earth’s history.
This means it is quite possible that the rebels were exiled at a time when human communication technology was not as pervasive and powerful as it is today, allowing a relatively small number of powered individuals to exist among the human population relatively unnoticed.
Perhaps this intermingling of exiled Inhumans and humans is why so many Inhumans were revealed through Terrigenesis after Terrigen got into the fish supply. The arrival of exiled Inhumans on Earth would add a fresh influx of Inhuman genes to humanity’s gene pool.
Jiaying, Daisy Johnson’s mother, is the oldest Inhuman to appear on Agents of SHIELD, save for the demigod Hive. She founded the Inhuman refuge called Afterlife and managed to synthesize new Terrigen crystals to allow certain Inhumans to undergo Terrigenesis.
Perhaps she wasn’t alone in all of this. It is possible that she sought out others like herself after discovering her own abilities. Perhaps she found some of the exiles or their descendants and learned from them some of the secrets of Terrigen. Perhaps some of these exiles or their children are even among those that Jiaying stole life from in order to extend her own.
Its all theory, and it's mostly background color, but those exiled Inhumans did have to go somewhere, and its hard to see where else Jiaying would have found information on Inhumans if even much of Hydra was in the dark at the time.
Marvel’s Inhumans airs Fridays at 9 p.m. ET on ABC. Marvel’s Agents of SHIELD will return for its fifth season on December 1, 2017.