Book IV, fair warning, is kind of all over the place. The first story concerns Merlin and Nimue.
After Pellinore rescued Nimue and brought her back to Camelot at the end of Book III, Merlin started hanging around her kind of a lot. Malory gives us zero reason to think she was in the slightest into him, or even sending mixed signals. He pestered her in what we might charitably call a well-meaning way: asking about her background, offering to show her some magic tricks and so on, but man, he just wanted in her pants. Everybody knew it. Nimue, who was a Lady of the Lake (not the Lady of the Lake, whom Balin murdered, just a Lady of the Lake), she put up with him. She was absolutely willing to learn magic, but the sex part, not so much.
So one day Merlin took Arthur aside, and he started in on his longwinded and dire prophecies again — Launcelot and Guenever, Mordred’s coup d’etat, all of it.
Arthur cut him off. “Merlin, what’s this really about?”
“I’m going to leave soon,” said Merlin. “Boom, into the earth with me.”
Arthur threw his arms up in confusion. “So, first off, I know you’re like eight million years old, but aren’t you immortal? Also, ever since you went back on the you-get-tossed-in-a-ditch thing, I haven’t really paid attention to your prophecies.”
“It’s going to happen!” Merlin insisted. “Nimue is going to get tired of me pestering her, and trick me into going into a magic cave. Then she’s going to seal the cave up, and that’s it for Merlin.”
“Hmm. Sure. I’ll play along. Listen, I have an idea,” said Arthur. “Assuming for just one hot second that you’re right about what will happen if you do that. How about, instead of doing that, you don’t do that?”
Merlin scoffed. “You don’t know how prophecy works, do you?”
Arthur rolled his eyes. “Clearly I don’t.”
“You keep that magic scabbard, all right? Just remember that one thing. Keep the magic scabbard, and don’t let a woman you trust steal it from you. For all women are faithless, and men deserve no better.”
“Okay, you’re being even more gloomy than usual, Merlin.”
“You’re going to miss me when I’m gone!”
“Sure, sure. You want to meet for pancake breakfast tomorrow?”
But Merlin didn’t stick around for pancake breakfast! Instead he learned that Nimue was planning on leaving Camelot and invited himself along.
“Okay. Okay. Tell you what,” said Nimue. “You can come with, if you promise not to do any magic on me.”
“Done,” said Merlin.
“No magic at all. Because it’s occurred to me there are things like, I don’t know, charm person, or suggestion, or pervy enchanted sex slave spell. Magic that a freak wizard-soothsayer of your stature might have access to. I’m not into that.”
“No problem,” said Merlin.
“I mean, I can see how some might find it kinky.”
“But no, okay? Straight-up no.”
“Right,” said Merlin.
So Nimue and Merlin took a trip together. They went to France, and then from there to Benwick, and the court of King Ban. At this point Ban was off fighting Claudas, so Nimue and Merlin were instead put up by Elaine, Ban’s wife, the Queen of Benwick.
“Merle, hello!” said Elaine. “This was with Claudas has been hell, I tell you what, but welcome to Benwick! Who’s your lady friend?”
Merlin introduced Nimue and Elaine. “Are you the Elaine from Book I, Arthur’s middle sister whom no one ever talks about?” asked Nimue. “Gawain’s mother Margawse is the eldest, Morgan le Fay the youngest, and you in the middle?”
“No, that’s a different Elaine apparently,” said Queen Elaine. (ELAINE 3!)
“Well, I’m confused then,” said Nimue. “Clearly you’re a lady, not a knight. And if you aren’t related to Arthur, and you aren’t schtupping Arthur or Merlin, then why has Malory bothered to give you a name?”
“Let me answer that,” said Merlin, “by introducing you to Elaine’s son, Launcelot.”
Launcelot was the best, you guys, you don’t even know. You don’t. You might think you do, but, man. Malory loves him some Launcelot. Of course at this point of the story Launcelot, prince of Benwick, was just a kid. An awesome kid, but still.
Merlin clapped Li’l Launcelot on the back. “Elaine, I know this war with Claudas isn’t going well, and maybe you think Arthur should send some military support what with the alliance against Team Lot & Mister 100 and all, but you just wait. In twenty years Launcelot here will be basically the best knight ever and he’ll win the war against Claudas, I promise. This a Merlin Solid Lock-of-the-Week prediction. Also his name is Launcelot.”
“I know his name is Launcelot. Merle, you’re not making a whole lot of sense,” said Elaine.
“I’m just demonstrating how Merlin I am,” said Merlin. “You know they say my mother was wed to the devil?”
“I have heard that,” Elaine admitted.
“Merlin, c’mon, you’re being weird again,” said Nimue.
“One more thing!” said Merlin. “Then we can go visit Cornwall and finally be wed!”
Nimue visibly shuddered at the thought, but Merlin was on a roll.
“Now Launcelot, here, that’s actually his middle name…”
“That’s true,” Elaine said, impressed. “We call him Launcelot, but his first name…”
“His first name is Galahad,” said Merlin.
“Wow! Guessing my son’s first name! That’s quite a trick!” exclaimed Elaine.
“That is, like, the least impressive trick I have seen Merlin do in the last month,” said Nimue.
“So your prophecy is that he’ll grow up to be a great knight?” Elaine asked Merlin.
“The greatest!” cried Merlin. “And to answer the question you’re about to ask, yes, you get to live to see it.”
Afterwards Merlin and Nimue did go to Cornwall. Merlin wanted to fool around, but Nimue was very plainly not into it. She kept trying to ditch Merlin, and he kept using his magic powers to show up again: she would duck out of the back of nightclubs, she would check out of hotels in the middle of the night, that sort of thing, and every time she turned a corner, there he’d be.
He wouldn’t leave her alone or take no for an answer. He didn’t deceive her or force himself on her, so, one up on some people Uther, but Nimue didn’t have a very good time while he was around. She wouldn’t change her mind, and Merlin refused to take the hint. One day, they were out touring the various magical lakes of Cornwall, like you do, and she tried to explain.
“Listen, Merlin, I have a great deal of respect for your incredible magic powers, many of which you’ve taught me.”
“But also I find you weird and gross. And you’re the spawn of the Devil.”
“Where are you going with this?”
“I’m saying I don’t want to sleep with you.”
“Huh.” Merlin turned away from her. “Check out this rock,” he said, pointing.
Nimue didn’t want to change the subject. “Merlin…”
“It’s a magic rock!” Merlin snapped. “It’s very impressive actually. You can go inside it. There’s a magic cave in there.”
“Yes, of course. I just said. And while you’re in there you wouldn’t want anyone to put a stone on top of it and seal up the entrance that way,” Merlin continued.
“Anyone in the rock when something like that happened, well, they’d be trapped for the rest of this novel at least. You can’t cast teleport without error to get out of it, or plane shift either.”
“So, yeah.” Merlin stood there, and looked at the rock, and back at her. “You want to go in and make out in the magic cave?” he asked, sounding more resigned than hopeful.
“No.” Nimue bit her lip. “But I, uh, I guess would sort of like to see the inside of a rock? I’ve never been in a magic cave. It sounds pretty… magical.”
“Oh, it is,” Merlin agreed wearily.
Nimue took a deep breath. “You go first, though. So I can see how you get in there.”
So Merlin magicked himself into the rock, and Nimue grabbed a stone and stuck it on top of the rock, and then she breathed a sigh of relief because she was finally free of the creepy old man who’d been bugging her all this time.
And that’s it for Merlin, barring a few continuity errors and flashbacks! Farewell, Merlin!