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.
Nightmare Fuel: Reading the final book, it seems that at least part of why actual displays of full-force magic are so rare in the series is that Prydain-style spellcasting results in this when fully unleashed.
In Taran Wanderer, Morda transforming the protagonists into animals is plenty creepy. Doli, who was turned into a frog, seems to constantly be ill, spasming at one point, and nearly dies from lack of water. When the rest are caught by Morda, he creepily ties them all up before enchanting them one by one, gleefully musing to Taran about how he wants to pick a form for him that's particularly humiliating and easy to enslave. And when Taran manages to free himself and fight back? Morda used his magic to make himself unkillable. Even more scary, Taran only found Morda's Soul Jar completely by chance. If he didn't have it with him, not only would it have been impossible to kill Morda, but there would have been nothing to protect Taran from the ensuing attempted transformation spell after stabbing the guy didn't work.
Tear Jerker: A half dozen in the last book, easily. Among the highlights: Coll dies and is buried far from home in a barren wasteland. King Rhun is killed while leading a badass rescue mission. Fflewddur Fflam destroys his beloved harp and, as one of the Sons of Don, has to leave Prydain forever. Gurgi must also leave with him. Doli and the Fair Folk all withdraw to the underground realms, never to visit the surface world again. The list goes on.