Inverted with the Book of Three. It will only serve good people with honorable and honest intent.
Similarly, Gwydion's sword, Dyrnwyn, can only be drawn (safely) by those of "noble worth." Taran (and others) initially think this means only one of the Sons of Don (due to the fact that they originally mistranslated that part of the inscription as "Royal Blood"), but in a critical moment Taran himself is able to draw the sword when his intention is pure (to defend someone else, not to attack in anger) and after undergoing Character Development in the fourth book, where he developed the personal nobility to wield the sword without falling to evil.
Authority Equals Asskicking: Played straight, especially with Prince Gwydion and King Smoit. In a land like Prydain, you only become royalty by being able to kick everyone else's ass.
Averted, ironically enough, with Arawn. His greatest strengths were always trickery & deception.
Subverted every so often, depending on the situation. One notable instance is when Taran finds out about how two noblemen living in King Smoit's domain constantly are at each other's throats, causing damage to innocent civilians in the process. Smoit's usual solution is to try to knock sense into them and shove them in his dungeons. Taran suggests he try something different, since it clearly isn't working. His proposed solution centers around humbling them and showing them the severity of the damage they've caused (making them serve as laborers for the farmer whose land they destroyed) as well as finding a third option for what to do with the prize cow they were fighting over (give it to the same farmer, and have the cow's next, equally-valuable, twin calves be split between the two). Since the two noblemen are seen standing peacefully together at the end of The High King, it presumably left an impression on them.
As Long as It Sounds Foreign: A mild example. Nearly all of the character and place names are derived from Welsh mythology, but Word of God admitted to having completely made up the very Welsh-sounding Eilonwy.
Backup Bluff: King Rhun, during a fight where the air is full of smoke and confusion, races his horse from one side of the battlefield to the other, shouting orders to regiments of cavalry that don't exist.
Badass Beard: King Smoit is known for his long fiery red beard.
Bigger Bad: Possibly the mysterious lord of Gwyn the Hunter fits the bill.
Big Good: High King Math, son of Mathonwy, and Dallben the Enchanter.
Bilingual Bonus: Several of the names of people and places. For example, "Hen Wen" means "old white [one] (feminine)" in Welsh. Gwydion means "born of trees." Averted with Eilonwy, which (as noted above) Lloyd Alexander invented for the books.
Bittersweet Ending: Arawn is defeated, but Prydain has been ravaged by war, many heroes have fallen, the power of magic has been lost forever, and Taran and Eilonwy give up eternal life and are separated from nearly all of their friends in order to stay in Prydain to help rebuild. On the other hand, Taran became High King of Prydain, married Eilonwy, and they both led happy and fulfilling lives and were such prosperous rulers that the bards wrote songs about them.
The Blacksmith: Hevydd. Also, Coll. Hevydd teaches Taran to forge blades, but while Taran proves to be a talented smith, his heart isn't in it.
Doli and his invisibility powers, which make his ears ring and hurt.
Also Fflewddur's harp, which is of excellent quality (and enchanted), but it's almost always in need of repair, since the strings keep breaking every time Fflewddur stretches the truth.
The powers Eilonwy and her mother inherited provide the wielder with a lot of power, but they also did neither of the women any good. As an enchantress, Queen Angharad was expected to marry a man born with magical powers of his own, and ended up banished when she decided to elope with a man who she loved, but who gained his magical powers from his own efforts and not birth. Eilonwy, meanwhile, only uses a magic spell once in the series, which doesn't even really work. She was kidnapped from her mother as a baby because Achren wanted to steal her powers, and ends up abducted and brainwashed in the third book, because of this plan. After that, she can't even use her powers, though she still has them. The last straw is in The High King, when she learns that her latent magic is still enough to mean she has no choice but to leave Taran and travel to the Summer Country, which she declares as being "worse than unfair" because she never asked for said powers. When Dalben informs her of a way to be rid of them, she does it without a second's hesitation.
Break the Haughty: Taran is horrified to find out that his father is Craddoc the shepherd and views his new life as a prison sentence. His time working for Craddoc is what is truly responsible for convincing him that nobility comes from work and honor rather than royal blood and wealth. By the time Craddoc dies and confesses that Taran isn't really his son, Taran has acquired humility and is not ashamed to be a shepherd's son.
Cassandra Truth: Seriously, Fflewddur really is a king. Granted, his kingdom is so small that he can leave his palace in the morning and be out of his kingdom by the end of the day, but he's a king nonetheless. Unlike most things, he doesn't feel the need to exaggerate this and freely admits he doesn't like being in his kingdom, which is why he became a bard.
Catch Phrase: Almost every important character besides Taran has at least one.
"Taran of Caer Dallben, I'm not speaking to you!"
"Crunchings and munchings."
"Deliver us from fools and assistant pig-keepers."
Taran, who grows from a stubborn, witless child to a wise and noble leader.
Also Eilonwy, who starts out rather bratty and temperamental, growing into a mature and compassionate person by the end of the series. Some of her development is off-page, as she does not appear in the fourth book.
In fact, almost every major character gets some important development, but Taran and Eilonwy are the most noticeable since the books span their adolescence.
Changed My Mind, Kid: Doli walks out on the companions at least once a book, only to come back in short order, to the point where it becomes something of a running gag.
Changeling Fantasy: Zigzagged. Taran knows he's adopted from the start, and hopes he'll turn out to be a prince ... but when he sets off to find out who he his (by learning who his parents were), he gets more than one answer. In fact, though at least two men (a poor shepherd and a king) both try to claim him as a son, his real parents are unknown, even to Dallben.
The ring Eilonwy receives at the end of the first book ends up coming in quite handy in the finale.
Also, the gwythaint. When it finally returns the favor, the reader's probably already forgotten the incident. A Genre Savvy reader would note that she already rendered Taran and co. one favor, and that was all most of us would have expected.
A Child Shall Lead Them: Taran still seems to be a teenager when he is made a war leader, and eventually High King.
Civil War: Like historical Wales, the various kings and lords of Prydain are always fighting somewhere in the kingdom. In addition, several of these kings side with Arawn against Math for their own benefit, up to and including Pryderi, the most powerful ruler in Prydain except for King Math.
Clever Crows: The heroic Kaw the crow, who turns out to be an important ally to the heroes.
Cloud Cuckoo Lander: Eilonwy, while still managing to be the only person in the group with any common sense.
Gwystyl deliberately hides his competence and badassery so well that he can come across like a wiener while helping Taran and his crew break into a castle.
Fflewddur Fflam is the less than popular king of a tiny kingdom and a semi-talented bard who tells grandiose lies. By the end of the first book we find out that he is also a deadly swordsman and a tried warrior. Amusingly, a prequel short story implies that most of the time, Fflewddur himself is unaware of his own badassery.
Crown of Horns: In The Book of Three the chief villain is the Horned King, who wears a mask made out of a human skull with great antlers rising in cruel curves. He is a warlord who is Arawn's champion and the War Leader of Annuvin.
Curb-Stomp Battle: The second half of the Battle of Caer Dathyl. Pryderi's regular forces are marginally defeated, but the armies of Prydain can't stand against the Cauldron-Born.
Cursed with Awesome: Glew turns himself into a giant and gets trapped in a cave he's too big to maneuver through.
Deal with the Devil: NEVER make a deal with Arawn. Unlike most "devils", Arawn never keeps his end of the bargain.
Death by Newbery Medal: The High King won a Newberry Medal and is a veritable named-character bloodbath. Even if none of the characters on the cover die. No, not even the Really Big Cat. The Black Cauldron, by comparison, won a Newberry Honor and only has a body count of two.
Everything's Better with Princesses: Eilonwy. Somewhat subverted, in that Taran (and therefore the reader) does not learn that she is a princess until literally the last page of the first book — when Dallben mentions it casually. Eilonwy herself never drops so much as a hint, except when noting that "mine are the people of Llyr Half-Speech, the Sea King." Her royal heritage is a plot point in the third book, but otherwise, she never concerns herself with princessdom very much. Her informed princessdom is justified, in that she is Last of Her Kind, so in political terms her heritage is largely moot.
Everyone Can See It: The entire group seems to be aware of Taran's feelings for Eilonwy except Taran himself. Eilonwy herself lampshades this twice in the last book. Taran's crush on Eilonwy is fully developed by the middle of book 3; in fact, Achren uses it to torture him. What he has trouble figuring out is that, yes, the princess likes him back, pig-keeping and all.
Something similar happens to the Huntsmen, not long after. The heroes see them camping on the banks of a dried-up river and melt a frozen waterfall to flood them out. As soon as they climb to dry ground, a group of bears and wolves maul them to death.
Extreme Omnivore: It's never explicitly stated, but it's heavily implied that Orgoch (of the Three Sisters) eats pretty nearly anything, including people.
The Faceless: Arawn Death-Lord, partially because of his Shapeshifting talents. Menwy the Bard is the only mortal on record to have seen Arawn's true face. Considering that she trained him, Achren has probably seen his true face as well. She even states that she will know it's him no matter what form he shapeshifts into.
Fainting Seer: Hen Wen in The High King, including a combination of terrified refusal to pass on her visions, and bizarre, nonsensical prophecies before she goes into Heroic BSOD.
The Fair Folk: A bit of a subversion, as they have no particular liking for men but are willing to work with them against Arawn. The king is grudgingly fond of Taran and Eilonwy, and Doli has more affection for the entire group than he likes to admit.
The Ghost: During The Black Cauldron, Adaon speaks often and fondly of Arianllyn, the girl to whom he is betrothed. She's never seen; in fact, the only thing the reader ever learns about her is that she was the one who gave Adaon his brooch.
The Grim Reaper: Although Gwyn the Hunter isn't anything like the hooded and cloaked skeleton we're all familiar with, he seems to have the same role as the Reaper in the setting. (He isn't evil, though; Arawn, the "Lord of Death," fills that role.)
"The black beast you saw is a harsh master; its claws are sharp. Yet I did not feel them until now."
Heroes Want Redheads: Eilonwy, whom the narrative notes has red-gold hair. A lot of artwork inaccurately depicts her as blonde, however, probably because of The Film of the Book. And when she is shown with red hair (see some editions of The Black Cauldron), it's pure red, not the more subtle shade that "red-gold" is probably supposed to describe.
Heroic BSOD: Taran, following the death of Craddoc the shepherd. Not just because Craddoc died, but that his first thought on seeing him was that he was finally free from his life as a shepherd. He was so ashamed and disgusted with himself for having ever had the thought that he never got over it or forgave himself for it.
Heroic Sacrifice: Ellidyr in The Black Cauldron,Rhun and Coll in The High King. Maybe Achren, depending on your point of view.
Honor Before Reason: A recurring theme. A major part of Taran's growth is learning when to put aside his honor.
Horned Humanoid: The Horned King. Somewhat subverted, as the horns are part of a helmet he wears, not a part of the Horned King himself.
Horse of a Different Color: Llyan, a small wildcat who grew to the size of a horse through the use of potions. She eventually adopts Fflewddur and allows him to ride on her back.
Humiliation Conga: Queen Achren. Starts off as a powerful sorceress and queen, and rules Prydain as an absolute tyrant. Later, she is overthrown by her more powerful protege and consort Arawn, and is moved to Spiral Castle. The castle is destroyed by the heroes, robbing Achren of her powers, which she attempts to replace by draining Eilonwy. She is also repeatedly spurned by the object of her affections, Gwydion. By the fourth book, she's a powerless refugee who works as a maid in Caer Dallben. Quite a long way to fall.
I Know Your True Name: Used by Gwydion to defeat Horned King, but only in the background. A bit of Fridge Brilliance, when this somewhat out of place logic is applied to the rest of the series. Gwydion claims that naming something is to imply mastery over it. No one knows Taran's true name, thus making him master of his own destiny.
Implacable Man: Arawn employs entire implacable armies. The Cauldron-Born cannot be slain by any mortal craft and carry out their tasks without tiring and without question or remorse. The Huntsmen of Annuvin are feared and renowned for pursuing their prey relentlessly, and fatigue means little to them. They can be killed, but that just makes them mad.
The major disadvantage of the Cauldron-Born is that they become weaker the further they are from Annuvin.
The strength of the Huntsmen is that when one is killed, the rest of them become stronger.
Informed Ability: Many characters are said to be enchanters, but they rarely if ever use these abilities. Possibly justified with Eilonwy as she never finished her training. And she destroys any hope of mastering her magic in the third book.
It Is Pronounced Tro-PAY: Not actually an example of this, but if you're not familiar with Welsh and its charmingly quirky spelling, you'd be fooled.
What doesn't help is that Welsh has seven vowels, each with two or more pronunciations - A, E, I & O sound roughly like you'd expect, but U sounds like I in all aspects and W is pronounced 'oo', in various ways. And Y has four pronunciations - it normally sounds like an I or a U in the last syllable of a word, or 'uh' elsewhere. This is actually quite hard to forget. No, really.
In the first book, Gwydion finds Taran to be so troublesome that he gives serious consideration to just dumping him back at Caer Dalben. Unfortunately, he must hurry to find Hen Wen (also, there'd be no story if that happened).
Kill 'em All: Many, many characters died in the last book, The High King. Perhaps for this reason it's left off many a school reading list that contains the other four books in the series. This is despite the fact that it won a Newbery Medal...
The closing body count at the end of the book is: Coll, Achren, Magg, High King Math, Annlaw Clay-Shaper, Llonio, King Rhun, King Pryderi, Arawn, Achren, and Taran's gwythaint. If you count returning to the "Summer Country" as a metaphor for death (which it is in some belief systems), then the book also claims Gwydion, Fflewddur, Llyan, Doli, Taliesin, Gurgi, Glew, and the Fair Folk and Sons of Donwrit large, since following Arawn's death The Magic Goes Away, so to speak.
King Incognito: Prince Gwydion and Fflewddur. Gwydion travels around the countryside in common garb because he doesn't buy into the "clothes make the man" cliche; in The Castle of Llyr, he is deliberately disguised to avoid detection. Fflewddur does much the same, only he goes out of his way to remind everyone that he's a king.
Last of Her Kind: Eilonwy, the last descendant of the royal House of Llyr. As the last Princess of Llyr, she alone is heir to a sizeable number of enchantments and magic powers, which reside in her by birth; but because her father was a non-magical commoner, she tends to refer to herself as being only "half an enchantress". This only really becomes relevant starting from the third book in the series, when she is returned to her ancient family castle, Caer Colur; having been kidnapped as an infant, she never realized the scope of her magical heritage.
Left for Dead: Happens to Gwydion with surprising regularity, usually allowing him to go off and do something just as or even more badass than Taran's group elsewhere.
Lie Detector: Fflewddur's harp, but it only detects Fflewddur's lies.
The Load: Taran himself in book one. Rhun in book three. Glew in book five.
Load-Bearing Boss: Half the architecture in Prydain appears to be held up by villains.
Spiral Castle collapses after Achren is defeated in The Book of Three, but it was more of a happy accident. Taran pulling Dyrnwyn (a load-bearing sword) from the tombs in Spiral Castle lead to its collapse—it just happened to perfectly coincide with Gwydion warding off Archen.
Caer Colur collapses after Achren is defeated in The Castle of Llyr.
In a non-villainous example, Dallben claims that his home will be consumed with magical fire if he is killed, along with anyone foolish enough to kill him. He may have been bluffing about this, we never actually find out either way.
In The High King, Annuvin is destroyed when Arawn is slain.
Following Arawn's death at the end of The High King the Fair Folk, the Sons of Don, and other magically-adept humans like Dallben use this as an excuse to leave Prydain, claiming they're no longer needed. Eilonwy gives up her powers rather than leave Prydain out of love for Taran.
Now now, only "the enchantments of evil" have been vanquished.
Dallben gives the breaking and complete loss of Hen Wen's oracular powers as further evidence that the good magic also has to leave the mortal world.
Even Dyrnwyn loses its powers after Taran slays Arawn with it.
Magic Mirror: The Mirror of Llunet is the object of Taran's quest in Taran Wanderer. Taran wishes to know of his parentage and so seeks this mirror which will show the truth.
Monster Shaped Mountain: The High King. Mount Dragon was so named because its peak was in the rough shape of a monstrous, crested dragon's head with gaping jaws, and on either side the lower slopes spread like outflung wings.
Motor Mouth: In the first three books, Eilonwy talks so much that it annoys the villains to the extent that, on an occasion when everyone else is merely tied up, Eilonwy is Bound and Gagged; in the fifth book she's a bit more subdued.
Nigh Invulnerable: The Cauldron-Born, and Morda. At least until their respective weaknesses are discovered....
No Man of Woman Born: A prophecy states that the Big Bad will be vanquished only when such things as "rivers burn with frozen fire" and "night turn to noon" occur. Some characters set a fire to melt a frozen waterfall and the burning logs are carried on top of the ensuing deluge, while another uses magic to light up an entire valley in the middle of the night.
Offstage Villainy: Sure, his subordinates and armies are out in full force, but Arawn himself just can't be asked to actually do anything until The High King. However, he does his own dirty work in one of the prequel short stories.
Orcus on His Throne: Arawn, despite being the nigh-omnipotent "Death Lord", works primarily through proxies like The Horned King, Morgant, Magg, Achren and Pryderi and leaves Annuvin just once to steal Dyrnwyn. It's Justified in that Arawn can be killed when he leaves Annuvin and takes a mortal shape, and would rather not risk his own life when he has a horde of Huntsmen, gwythaints, deathless Cauldron Born etc. Arawn is also portrayed as more of a trickster or Old Scratch figure who prefers to get what he wants through manipulation and guile rather than overt shows of force.
Out-Gambitted: Pryderi thinks he is tricking Arawn into serving him. He isn't.
Papa Wolf: Gwydion can get this way when his companions are threatened. (He gets really mad when Achren torments Taran in The Castle of Llyr.)
Parental Marriage Veto: Eilonwy's parents. Gwydion mentions it briefly in the third book; the extra volume The Foundling and Other Tales of Prydain gives the story of their romance in much greater detail. In summary: Angharad, Eilonwy's mother, was a highborn enchantress who rejected her snooty wizard suitors and instead fell for a humble storyteller. (Lloyd Alexander does love his bards.) The pair of them were exiled.
Patronymic: In Prydain, men use patronymic naming, and women use matronymic naming. Taran's lack of such a name troubles him greatly and is a driving force for much of his story, particularly in Taran Wanderer.
Positive Discrimination: Eilonwy at Taran's expense in the first three books. Although she herself is pretty shrewish at the beginning.
Prince Charmless: Ellidyr. Slightly subverted in that there's absolutely no potential for romance between him and Eilonwy. In fact, he actually goes out of his way to insult her quite a few times in The Black Cauldron, introducing the reader to Taran's Berserk Button in the process.
Running Gag: A string on Flam's harp breaks whenever he lies (which is often). The bigger the lie, the more strings break.
Sand in My Eyes: In The High King, when Fflewdur Flam sacrifices his harp for firewood, he complains of how it smokes, though it burns with very little smoke. He then comments that he's glad to be rid of it, and two strings break.
Scaled Up: Arawn does this just before his death. Justified as he was trying to get out unseen, and almost succeeded.
Schmuck Bait: The Book of Three. Taran, what is your fascination with this mysterious and forbidden tome?! Fortunately, the book seems to be sentient, and it punishes Taran's innocent curiosity with the equivalent pain of a few mere bee stings. King Pryderi, on the other hand, wanted to steal the book and use its secrets to gain power, and he was not so lucky.
Reading into the backstory a bit, turns out the Book of Three was a bit of Schmuck Bait for Dallben, too. Evidently in Prydain, wisdom comes with a high price. It turned Dallben from a youth to an old man overnight.
Self-Disposing Villain: King Pryderi. To elaborate, he attempts to steal the Book of Three from Dallben. The enchanter warns him that betraying his former allies and working with Arawn have marked him for death if he touches the Book. He does it anyway and the Book burns him to a crisp with a lightning bolt.
Sense Loss Sadness: After giving away Adaon's brooch to the three witches, Taran notices he lacks the clarity and wisdom he had while he wore it.
Small Name, Big Ego: Ellidyr treats the main characters with much contempt and responds violently if anyone so much as thinks about impugning his honor. It's explained that he is like this because he is from an old yet impoverished noble family, and his father and elder brothers squandered their house's money and good name. Poor Ellidyr hasn't got much left except his pride and his horse.
Solar and Lunar: The emblem of the House of Don is the sun, on account of the fact that the Sons and Daughters of Don are descended from the Lady Don and her consort, Belin the sun king. Meanwhile, the emblem of the House of Llyr (Eilonwy's all-but-extinct lineage) is the crescent moon; this is on account of the fact that they are descended from Llyr Half-Speech the Sea King, and the tides of the sea are governed by the phases of the moon.
Taran harbors some unfortunate ideas about women in his youth. Some time spent as the apprentice of the spry Dwyvach Weaver-Woman helps him learn better.
Many characters wish Eilonwy would stick to women's tasks, out of concern for her safety. Naturally, she will not hear of it.
Stone Soup: How cooking works in Llonio's household in Taran Wanderer. He sends all his children to find ingredients, and whatever they bring back ends up in what can only be described as a sort of pancake-omelet.
Sweet Polly Oliver: Eilonwy is forbidden to follow the men into battle in The Black Cauldron because she's a girl, so she dresses as a boy in order to fight alongside Taran. By the time The High King rolls around, nobody really cares anymore because she's proven that she's a very competent fighter (and because they have learned that they just can't make her stay home). The fact that she has latent magical powers doesn't hurt anything either.
Tag Along Kid: Taran starts out as this to Gwydion but quickly proves his worth. Later, Llassar becomes this to Taran but he too proves himself quickly.
The Unreveal: We never learn who Taran's biological parents are. They're probably nobody we know. Also, we never really learn who/what Arawn really is. After he's killed, he reverts to his true form which ends up lying face down on the floor, but before anyone can go near him, his fortress starts to crumble and everyone has to split.
Though with Taran, the fact that it's not revealed, and in fact probably can't be revealed, is part of the point. He's not just the son of any two people, but all of Prydain, noble and common alike.
Unusual Euphemism: Dorath threatens Eilonwy by claiming he will "remove her charms". Hey, it's a kid's book after all.
Vague Age: Everybody, really, but it's most notable with Taran and Eilonwy. It's generally understood that the series begins when they're prepubescent and ends around the time they reach adulthood; the only other clue to their ages is that Eilonwy is one or two years younger than Taran. In the last book, Dallben even keeps it deliberately vague by mentioning an event that happened around Taran's birth as having happened "as many years ago as you yourself have years."
Vain Sorceress: Achren, who according to Eilonwy, "loves jewelry, but it doesn't become her one bit."
Wandering Minstrel: Fflewddur's (largely unsuccessful) career as a bard before meeting the heroes. However, it was his own choice, since he finds being a failed bard much more enjoyable and fulfilling than staying in his dismal little kingdom.
What's Up, King Dude?: Back in Fflewddur Fflam's kingdom, children would often play games and sports in his throne room because of ease of access, and they knew that he was far more likely to join in their games than to shoo them out of the castle.
Dragged out until the very last page of the final book with Taran and Eilonwy. They Do, of course.
Eilonwy lampshades the whole situation with her response to Taran's marriage proposal: "Well, indeed. I wondered if you'd ever get round to asking. Of course I will, and if you'd given half a thought to the question you'd have already known my answer."
Wise Beyond Their Years: Taran, starting in book four, but by this point he's pretty well earned it by making numerous painful sacrifices.