History
Blackagar Boltagon is an Inhuman. He was subjected to the mutagenic Terrigen Mist while still an embryo, gaining strange powers surpassing even the Inhumans' norm. After his parents died, Black Bolt became the king of the Inhumans.[1]
Infinity[]
When Thanos arrived on Earth along with his Black Order, Black Bolt helped S.H.I.E.L.D.'s Alliance to repel the invasion after it was discovered that Thanos' demand of the tribute of the heads of every youngling between the ages of sixteen and twenty-two, or the annihilation of Earth, was a cover for the actual search of someone called Thane, whose exactly identity was unknown even for the Mad Titan, but his location was apparently New York City.
Black Bolt contacted the heroes and offered them his help, still not telling what did he know. The person Thanos was looking for was revealed to be his lost Inhuman son, Thane, who decided to join the fight against the Black Order after deciding Black Bolt had protected him enough.
Thanos and the Black Order withdrew after a final confrontation against Black Bolt. Blackagar was offered a place in the Alliance's ranks, and he accepted.[1]