If you’re like me, you’ve read the Return of the King more than once, but never made it all the way through the appendices. There are six appendices, but the one we’re going to be spending the bulk of our time on (13 of the 15 sections) is Appendix A, a grab-bag of history and narrative. Expository backstory, ho!
THE FIRST (OF THREE) AGES OF HISTORY, SUMMED UP
The story of everything important starts, of course, with the silmarils. See, there once, in the Undying Lands off to the Uttermost West, full of Valar (angels) and Eldar (elves) there was this elf named Feanor, who decided to create the best treasure ever, and he did, because he was awesome at that. Way back in the day, before the invention of the sun and the moon, light came from trees (two giant trees) and Feanor took some of the light and put it in three jewels, the silmarils.
The silmarils were awesome, oh, you can’t imagine. They were so, so pretty. Take every art object that’s ever existed, and put them in a big pile, and light that pile on fire, and the resulting fire still isn’t going to be nearly as pretty as the simarils. Just hearing about them, you want them. You know you do.
Morgoth wanted the silmarils. We don’t get an explanation for Morgoth just yet — all we need to know is that whenever anyone in the Lord of the Rings mentions the Enemy, as in “servants of the Enemy,” generally they aren’t talking about Sauron, Sauron is just Morgoth’s sidekick. Morgoth is the real Enemy. It’s like a post-credits twist!
Anyway, Morgoth steals the silmarils and destroys the Two Trees while he’s at it, which was a whole big thing. He holes up in a fortress in Middle-Earth, which at this point in time is a big mostly empty continent to east of the Eldar, the important/civilized/Elvish people.
Now, the angels who ruled over the elves were like, just let it go, but Feanor couldn’t let it go, so he gets a big pile of Elves together and leaves the Blessed Realm, with the intention of conquering Morgoth’s fortress by force. Once he and the other Elves get to Middle-Earth, they meet the Edain, which is a particular ethnic group of Men, and they try and fail to get the silmarils back from Morgoth. It’s a big long thing.
Gear shift: in the history of Middle-Earth, there are exactly three occasions wherein one of the Eldar and one the Edain hooked up and had kids. You’ve got Beren and Luthien, you’ve got Idril and Tuor, and you’ve got Arwen and Aragorn. Arwen and Aragorn you know all about already, unless you haven’t seen the movies and just read the books, because it is extremely easy to fail to notice that Arwen is in the books at all, but I digress.
Luthien was the daughter of Thingol, a major Elf king of a place called Doriath, and her mother was Melian, which is funny because Melian was an angel rather than an elf. Beren was some guy, and together the two of them managed to get one of the silmarils from Morgoth. Luthien chose to become mortal, which apparently she could do, and she and Beren had a couple of kids and eventually died.
Idril was the daughter of Turgon, another major Elf king of a place called Gondolin (aka where all the best magic swords came from, Thorin’s sword Orcrist and Gandalf’s sword Glamdring and Bilbo’s sword Sting). She hooked up with another guy, Tuor, and they had a son: Earendil the Mariner.
Earendil married Beren and Luthien’s granddaughter, who inherited the silmaril from them, and it was Earendil who went from Middle-Earth back to the Undying Lands to ask the angels and the elves who never left to help Feanor’s people and the Edain defeat Morgoth once and for all, which, they did, and that was the end of the First Age. There was a big world-shattering apocalypse and a lot of land got sunk.
The two silmarils that Morgoth still had were lost, and the one that Earendil bore back to the Uttermost West stayed there or became a star in the sky or something, anyway, the point is that the silmarils were all gone by the beginning of the Second Age.
There’s a lot more about the First Age in the Silmarillion, the invention of the sun and anti-light (which is like darkness, but worse) and so on, but this is all the ROTK has to say about the First Age.