The Shadow Men are entities unbound by time and space (who have a connection with Korrok & Millibutts, which we'll get into later). They can control time & space, being able to retroactively rewrite reality on a whim and move between universes and time at will. Shadow Men live inbetween moments and outside time, across dimensions and maybe never in any particular place or time at once. That they look like men is humans enforcing an image on something they cannot imagine. They're also thought of as ghosts, and long ago, Shadow Men would have been called faeries and succubus in the past, and would appear as tiny gray aliens to others. As a threat, a Shadow Man causes the airplane Amy was in to crash and not crash, and telepathically communicates with Dave (and seems to move Dave to this explosion). Shadow Men exist everywhere, similar to electricity, and encounters are rare, causing you to lower your guard, until you actually encounter them directly... A false child larvae...