When Legion, the mentally unbalanced mutant son of Professor Charles Xavier and Gabrielle Haller, traveled back in time in an attempt to kill Magneto, he accidentally killed Xavier instead, creating a splinter universe known as the Age of Apocalypse. In that reality, mutants became the dominant species of the planet. Apocalypse rose to power enslaved humanity.
The original Blackagar Boltagon was killed by his brother Maximus, alongside the entire Inhuman Royal Family. Maximus used his brother's corpse to create a clone, which he exposed to the Terrigen Mists. The Mists gave the clone powers similar to those of the original Black Bolt, though far weaker. The Mists also mutated the clone into a monstruous aberration under Maximus' mind control. In control of the Inhuman kindgom, Maximus bought his way into the servitude of Apocalypse, becoming the Horseman of Death. However, Death planned on taking the place of the High Lord.
When the X-Men appeared on the Blue Area of the Moon believing they would find Apocalypse, they faced Death and his Inhuman strikeforce, with Black Bolt among them. Black Bolt sparred against Sabretooth, but was taken out by Nightcrawler.