Long story short: Black Bolt detonated a massive bomb of terrigen to both advance his race and fuck over Thanos. Terrigen isn't lethal in the comics, though extreme exposure to humans can cause them to mutate and die shortly after. Terrigen can also restore mutant powers, however the powers will be altered significantly and can be extremely difficult to control. Unus the Untouchable, for example, could generate an unbreakable forcefield. After he lost his powers, they were restored by terrigen, however they functioned differently and he couldn't control his forcefield and accidentally killed himself (the air couldn't touch him so he couldn't breathe). Quicksilver had his powers restored and while he couldn't move fast like he used to, he gained the ability to travel through time