It's been so long that I can't remember my initial impressions, but I'm pretty sure it's a toss-up between the Lady of the Green Kirtle and Tash.

Tash--so you have the Calormens essentially repeating the Telmarine invasion. When that happened, Narnia as we know it hunkered down to weather the storm and then Aslan came to reawaken and revitalize Narnia and all was well. But this time Tash shows up and takes that hope of hiding away and... things don't go like last time when Aslan came roaring in to defeat the White Witch.

The Lady of the Green Kirtle is more subtlety terrifying--though I would not want to face a snake that's bigger than I am under any circumstances--but the loss of hope she brings with just a few words of her enchantment is scary.

Walking down the hall in the Magician's House when everything is invisible is nerve wracking, but that's not a creature, so it doesn't count.

In the books, I am not sure. If we include the movies, definitely the Sea Serpent!! That thing was nasty and kept changing.

I think I'm definitely going to go with the werewolf in Prince Caspian. He gives me the creeps.

A dull, gray voice at which Peter's flesh crept replied, "I'm hunger. I'm thirst. Where I bite, I hold till I die, and even after death they must cut out my mouthful from my enemy's body and bury it with me. I can fast a hundred years and not die. I can lie a hundred nights on the ice and not freeze. I can drink a river of blood and not burst. Show me your enemies."

Would not want to meet that chap in a dark alleyway. *shudders*

I think I would go with Tash. He sounded like the demon that the Calormenes accused Aslan of being. And I particularly think of Rishda Tarkaan's response, he who had not believed in his own religion's deity, and found that he was mistaken.

It would appear that Dark Island and Tash shared some characteristics--both were brought about by one's own command. Rishda called for Tash, Tash appeared. The dreams of Dark Island were each person's individual fears taking form.

Also, Tash did not seem to be destroyed. At least, not that we saw. Peter did not slay him, he only told him to go to his own place. To all intents and purposes, he was gone, I suppose. He was certainly not in the new Narnia.

If we include the movies, definitely the Sea Serpent!! That thing was nasty and kept changing.

That was a truly nightmarish design--much different than the single-minded threat posed by the creature in the book. (I always laugh when the book serpent noses the broken bits and pieces and can't figure out where the ship had gone.)

It's been so long that I can't remember my initial impressions, but I'm pretty sure it's a toss-up between the Lady of the Green Kirtle and Tash.

Tash--so you have the Calormens essentially repeating the Telmarine invasion. When that happened, Narnia as we know it hunkered down to weather the storm and then Aslan came to reawaken and revitalize Narnia and all was well. But this time Tash shows up and takes that hope of hiding away and... things don't go like last time when Aslan came roaring in to defeat the White Witch.

The Lady of the Green Kirtle is more subtlety terrifying--though I would not want to face a snake that's bigger than I am under any circumstances--but the loss of hope she brings with just a few words of her enchantment is scary.

Walking down the hall in the Magician's House when everything is invisible is nerve wracking, but that's not a creature, so it doesn't count.

Certainly. That is a good point. Because as you put it, Tash was fearful in one way, but the lady of the green kirtle tried to make them loose hope.