"A vicious monster (now) stalks you from the shadows"

It's not a matter of grammar, as both sentences are correct. But the second sentence sounds better:

Version 2: "As if breaking out of prison wasn't hard enough, a vicious monster now stalks you from the shadows."

Both sentences convey the same general meaning, but using now really emphasises the sense of ongoing struggle - "first this, now that".

They have slightly different meanings, IMO.

The first says that the stalking monster is currently stalking you (and may have been stalking you for some time), and is likely to cause you a problem very soon indeed.

The second says that the stalking monster has only just started to stalk you, and so you may have a few moments before it becomes an immediate problem.