Who is Robert "Bob" Reynolds, aka Sentry, the villain of the MCU's Thunderbolts, and how is he connected to The Void?

It's called the Void and presents itself as a vast black malevolent force with tendrils strong enough to casually break every bone in the Hulk's body, the ability to control the weather and create ...

and infini tendrils, which upon stabbing people can traumatize them with horrific visions of the past, present and future. The post Thanos vs. Sentry/Void: Who Is Stronger & Would Win in MCU?