Bob can manipulate matter right? What if Void uses the power to manipulate to manipulate his/its form into something monstrous?
Imagine seeing this average guy being consumed by shadows, the now inky eyes of his reflecting pinpricks of light with no visible facial features, light outlining his/its standing silhouette. Thinking your mind is playing tricks on you as blackness pools out from below his/its feet.
Void could move as fast as Sentry but instead of looking similar to teleporting behind, it could melt into the floor and emerge from, for instance, another persons shadow. The only tell being a darker than average shadow before it strikes (shadows don’t have eyes and there’s nothing to reflect off so no, no eyes sadly).
It could also stretch itself through shadows to grab like shadows in Princess and the Frog!
In that gif the shadows dragging another shadow instead of a person but I hope you get the jist.
And then, of course, physical form manipulation!
Unlike in the GIF, Void would likely keep the humanoid shape and alter itself to intimidate or unnerve: enlonged limbs, more light reflecting the eyes (altering shape of the eye to be more flat instead of curved). The movie is unclear on if Bob’s bones can be broken or if his flesh is simply unbreakable but I want to say enlonged teeth since the wetness of saliva makes parts of the bone reflective/seen easier. But that’s just a want of mine. The more predatory it looks, the more intimidating.
But sadly, I doubt this would happen. Void is Bob after all, and thus is not an entity completely separate from Bob and his desires. He isn’t a villain and wouldn’t want to remotely hurt anyone unless hurt or threatened.