Book 1, chapters 26-27

King Rience of northern Wales, all of Ireland, and "many isles", sends a message to King Arthur. He says that he's defeated eleven other kings, and then forced them all to shave off their beards. Rience then took those beards and made a mantle out of them! However, there's still a hole in the mantle, where Rience intends to sew in Arthur's beard, so now he demands that Arthur shaves for him. Arthur says a) he's too young to have a full beard yet (like seriously, how young is he?), and b) also, fuck you King Rience, you're the biggest asshole ever, so many fuck you:s, fuck you forever (it's a longer speech but this is the gist of it).

So that was chapter 26. In chapter 27, I fear we've got a bit of a continuity issue.

Merlin had previously neglected to tell Arthur about his biological family, resulting in Arthur knocking up his big sister completely unaware of the family relation (although totally aware that she was married, but whatever, that never stops anyone in this book) (also, serious cradle-robber, that big sister of his). Later on, Merlin tells Arthur that now he's gonna have an incest baby with his sister, who's fated to destroy him. In chapter 27, however, it's suddenly like Arthur hasn't heard this. Now, Merlin just tells him that there's a child born on May-day who's fated to destroy him eventually. Arthur resolutely thinks to himself that there's no kill like overkill, and in a probably Herod-inspired move gathers up every child in the realm born on this day, including his incest son Mordred, and put them all in a boat that is pushed out to sea so that they will die. Both less efficient and less humane than if he'd just had all their heads chopped off!
Obviously people are very upset about this, but most of them blame Merlin more than Arthur, which is, okay, fair enough I guess.

Anyway, the boat crashes on a shore by a castle, and all the kids die except for Mordred, who's saved by "a good man" who takes him in and raises him.

And that's the end of book one!

Book one, chapter 23-25 : Rome pays a visit, random fighting, Arthur receives Excalibur

Ok, so after a really long hiatus, for no reason in particular other than I had a lot of other shit to do, the Arthur saga is back. At this point, the super weird plot point with the Roman emperor is introduced.

Twelve knights arrive at Camelot, from Rome, telling Arthur he has to pay taxes. Arthur is like "Screw you, you can tell your emperor I'm coming down to pay him with my sword!"
Apparently, King Arthur was in an extra bad mood too, because Sir Griflet had been so badly wounded... in a fight he wasn't prepared for, and only engaged in because Arthur ordered him to.
You're weird, King Arthur.

Next day, Arthur is riding around on his horse, when he sees Merlin being chased by three murderous peasants. Was that a general problem, in these days? (Whichever "days" we're talking about, since there are so many anachronisms in this book?) Wizard-hating peasants? Arthur chases them off, and then tells Merlin that he just saved his life. Merlin is like nah, you totally didn't, I could have chased them off by magic any time if I wanted to, but I didn't feel like it. Merlin goes on to say that it's actually Arthur who's in danger, because he's soon gonna be in a fight with another knight, and soon enough they run into a knight called Pellinore, and Arthur fights him.

Pellinore is about to win the fight and kill Arthur when Merlin steps in and put a sleep enchantment on the former. Convenient! Merlin then goes into prophecy mode and tells Arthur that Pellinore is a really big knight (this is weird: Could "big" be used the same as "great" when this book was written? Or is he just literally very big? But if so, surely Arthur could see this with his own eyes, and wouldn't need Merlin to tell him?). He will have two good sons, Percivale and Lamerake, and one day reveal to Arthur the name of his own incest-son that he had with his sister (because, remember, Merlin hadn't bothered to tell them that they were actually siblings), who's fated to destroy the realm.

Unfortunately, Pellinore destroyed Arthur's sword in the fight. Merlin brings him to a lake, in the middle of which a white-clothed arm sticks up from the water, holding a sword. Merlin is like look, you can have this one instead!
So, I didn't quite get all this from just reading the book, I also did a bit of googling, but apparently the Lady of the Lake or the ladies of the lake (even though they talk about her in singular terms in this chapter, there are more of them later on) are like a kind of fairie species in English folklore. And they don't literally live in lakes, they live in a sort of magical fairy realm, but they put glamours over the portals making them look like lakes, which they ascend from when entering our world. So this lake lady who was holding the sword above the water now comes up and gives it to Arthur, on condition that he's gonna do her a favour later on when she asks. And Arthur agrees to this! He has no idea what the favour is gonna be or when she's gonna ask it. He's just "cool sword, sure, I'll do you a favour, whatever whenever."
Also, the sword has a magic scabbard, that makes Arthur invulnerable for as long as he carries it. Neat!