Many, many, many creatures of Magnamund (or the Daziarn, or the Plane of Darkness...) are absolutely nightmarish in appearance. Their descriptions are short but vivid, with nice details added (like how much they inspire revulsion to Lone Wolf) and with plenty quality illustrations, mostly from the protagonist's viewpoint (that is, more often than not, when the monster is about to gut you). And you get plenty of Red Shirts or Mauve Shirts' disturbing deaths, just to demonstrate how nasty those critters are.
- The Helghast◊ and Crypt Spawns, although already quite horrific, are hardly the worst of the lot.
- The Cener Druids in particular are dedicated to producing Nightmare Fuel. The Forest of Ruel is full of nasty things, and they're nothing compared to what's in the fortress of Mogaruith.
- The Rahkos from Book 7, a brain-eating, undead severed hand, is largely believed to cause the most Squick-inducing death...A searing pain explodes behind your eyes as the hand clamps itself to your head. As the decaying fingers pierce your scalp, forcing their way through your skull, your vision turns red and your body shakes uncontrollably. The hideous claw burrows deeper, feeding on the only source of nourishment that can sustain its existence: living human brain.
Your life and your quest end here.
- Big Bad Naar, the King of Darkness, manages to top his grotesque minions when you finally see an illustration of his favored form in Book 19. It's not so much the Body Horror, nor the eldritch puddles of black oil oozing from every pore... no, the worst part about Naar is that he has the eyes of a man.