How much of Ellidyr's actions towards the end of Book II are thanks to the corrosive effects of the Black Cauldron, and how much his own obnoxiousness?
Anticlimax Boss: Arawn; despite his build up, he just turns into a snake which is promptly killed by Taran in book five. Disney's version of The Horned King, who more closely resembles Arawn than the Horned King in the first novel (who was The Dragon to Arawn's Big Bad), also suffered from this.
Complete Monster: Arawn Death-Lord was once the evil master of Prydain before being driven back. Returning to power, Arawn uses the Black Cauldron to create the Cauldron-Born, nightmarish undead warriors caught between life and death, even if Arawn has to have hosts of living warriors killed to do it. Sending them to wipe out entire cities, Arawn plans on the complete conquest of Prydain and attempts to have one castle destroyed with all inhabitants to claim the lives of his destined foes.
Designated Hero: Taran's foolhardiness and obsession with his own glory in the first book particularly leads many readers to wonder why we are to cheer him on. He gets better.
Ensemble Dark Horse: Despite appearing in only a few scenes, the Horned King was notable enough to be promoted to the Big Bad in the Animated Adaptation of the first two books (which bears the title of the second, but draws about equally from both — which is to say, very little).
Evil Is Cool: The Horned King. He's mysterious, powerful, and wears an awesome mask. What isn't to like?
Genius Bonus: Aside from the many references, whether characters, places, or magical items, to Celtic Mythology and the Mabinogion, the moment at the climax of Castle of Llyr when Magg opens the gate to let the sea drown Caer Colur is likely a reference to either the legend of Ys or Wales' Cantre'r Gwaelod.
Heartwarming Moments: Taran wrestling with the offer to go to the Summer Country and have everything he's ever wanted, but turning it down because he feels the obligation to stay and fulfill his vows. Eilonwy immediately jumps ship to stay with him.
Fflewddur offering up his own harp as firewood to stop the heroes from freezing to death.
Ellidyr realizing how far he'd fallen and genuinely apologizing to Taran and the others. Then, understanding that words are cheap, proving his mettle by fighting his way through Morgant's warriors barehanded and throwing himself into the Cauldron to destroy it, at the cost of his life.
Taran's goodbyes to Coll and Dallben at the beginning of Book 4. Although none of them ever come right out and say it, it's clear that he loves both men deeply, and that not only Coll, but even grumpy old Dallben cares for him deeply in turn.
Hilarious in Hindsight: The protagonists are a farm boy, a cocky fighter, a wise master, an Action Girl princess, a snooty complainer, and a really hairy thing. Meanwhile, the main villain of the first installment is a tall, powerful, and mysterious warrior who fights on behalf of a being even mightier than he and terrifies all the protagonists (except the wise master) just by showing up. Yeah, this may have inspired a certain something.
Jerkass Woobie: Ellidyr. He spends pretty much the entire book being a jerk to everyone, but especially heaps scorn on Taran, going so far as to drag him bodily across Caer Dallben and take it for granted that Dallben would punish Taran for "impertinently" backtalking a prince. This reaches new levels around the climax, when Ellidyr uses the heroes' desperation to bring the Black Cauldron to Caer Dallben as leverage to force Taran into agreeing to basically act as his servant and pretend that Ellidyr was the one who found the Cauldron (which is doubly cruel considering that Taran only got the Cauldron himself by giving up a brooch that his dead friend bequeathed to him). When they all agree to these terms, Ellidyr goes on to lose his mind and try to kill them anyway, somehow convinced that they were going to double-cross him. However, over the course of the story, it's revealed that he's such a jerk because his family squandered their nobility and fortune, leaving him with virtually nothing but his title as a prince and his ill-tempered horse. He also ends up severely beaten and taken prisoner while trying to bring the Cauldron back to Caer Dallben, and is horribly broken when he is unable to even free himself to save the group from death. He ends up throwing himself into the Cauldron at the cost of his own life because that's the only thing he can do to help the others, and does so after swearing "on what little honor [he] has left" that he only wanted the Cauldron destroyed, and never intended to use it himself.
To make it even more of a Tear Jerker, said ill-tempered horse deliberately runs off a cliff in anguish at Ellidyr's death.
Only the Author Can Save Them Now: The Horned King in The Book of Three and the Cauldron-Born in The High King are each defeated by a Deus ex Machina that had not been pre-established. Gwydion's use of the Horned King's secret name - which he learned through a power gained from Oeth-Anoeth, which had also not been mentioned before. And the power of Dyrnwyn to destroy the Cauldron-Born also hadn't been mentioned prior to its use - everyone thought they were completely unkillable.
or Wales' Cantre'r Gwaelod