Earth X introduced a future where powers have become so commonplace that there is no longer a distinction between superheroes and regular humans. This universe -- dubbed Earth-9997 -- centers around the recently infamous Inhumans, and their king, Blackagar Boltagon, known better as Black Bolt. In Earth X, Black Bolt unleashes the the Terrigen Mists responsible for granting his people their powers upon Earth. The world then falls into turmoil when normal humans become superpowered mutates with amazing powers and it falls on Reed Richards, Black Bolt, and Franklin Richards to stop the growing threat of a Celestial growing in the center of the planet.
Earth X debuted in 1999 and was created by Jim Krueger, Alex Ross, John Paul Leon, Bill Reinhold, Todd Klein, Matt Hollingsworth, Melissa Edwards, and Linda Lessmann. The Earth-616 Inhumans are largely similar to the Earth-9997 Inhumans, and in both worlds, Black Bolt is in a constant battle against his mad brother, Maximus. Maximus planned to use a bomb he created to release the Terrigen Mist into the Earth's atmosphere, but the Inhuman Royal Family stopped him from detonating the bomb -- Maximus taking Lockjaw's life in the fight, which caused Black Bolt's wife, Medusa, to snap Maximus' neck in vengeance. It's then that Black Bolt looks at his family of Inhumans and grows tired of living in isolation from humanity, and decides to secretly fulfill Maximus' plan and release the Mist.
Black Bolt's powers revolve around him using his voice, so his character is almost always very demure and reserved. However, in this moment he shows how tired he is of living in the shadows in segregated continents far away from humanity. He desires a future where his family can walk with and be accepted by those on Earth, so the decision to force all of humanity to become Inhumans is a drastic, but understandable one. When the Inhumans return from fleeing to space, they see a world transformed by the mist and in chaos as humans are overtaken by their amazing powers.
After many heroes die in the resulting fight against the Celestials, Black Bolt makes the decision to sacrifice himself by using his voice power to travel across the universe to contact the all powerful Galactus. Black Bolt dies as he asks Galactus to save Earth by devouring the Celestial embryo inside the planet. Galactus agrees, and saves the Earth.
However death winds up being only the beginning for the king of the Inhumans. Since the conclusion of Earth X, the creative team has reunited to produce a number of sequels that feature the return of Black Bolt, albeit in a different form.
After his death, Black Bolt is seen in Paradise X as one of the many souls that roam the Realm of the Dead. He eventually joins Mar-Vell's fight against Death, who has been established as a villain in the series. With Black Bolt's help, Mar-Vell succeeds in killing Death, and he then creates a wonderful land called Paradise to replace the Realm of the Dead -- choosing Black Bolt to become one of the Avenging Hosts that protect the land and bring the dead to Paradise.
Black Bolt is presumably still protecting Paradise, but he has yet to make an appearance in the current Marvels X series, which focuses on the last "normal" human left on Earth, a boy named David, and the chaos that has erupted in the world around him.