Warning: Spoilers for The Witcher 3: Hearts of Stone lie ahead.
You might disagree, but to me, the title of most terrifying thing you encounter in The Witcher 3 goes to the Hearts of Stone expansion's Gaunter O'Dimm. Sure, he's a lot less in your face than a lot of the monsters and foes Geralt faces, but he's hinted to potentially be the most evil and powerful. Anyway, there's now a mod that sticks him in some swimming gear.
Yep, if you've ever been a bit unnerved by ol' Master Mirror's ability to pause time itself, cause shipwrecks in an instant, and recall the secret to making perfect gingerbread, it's ok, you can now slap some armbands on him to lessen his general spookiness. Or maybe this increases it? I'm having trouble deciding, to be honest.