The speech in the OP is Théoden's speech from the movie. In the book, he says,
Arise, arise, Riders of Théoden!
Fell deeds awake: fire and slaughter!
spear shall be shaken, shield be splintered,
a sword-day, a red day, ere the sun rises!
Ride now, ride now! Ride to Gondor!
and just a page or two later
‘To me! To me!’ cried Théoden. ‘Up Eorlingas! Fear no darkness!’
(ROTK book 5, chapters 5 & 6)
Now, Eomer does say something very similar to Theoden's speech from the Charge of the Rohirrim, upon discovering his sister and assuming her dead:
‘Éowyn, Éowyn!’ he cried at last. ‘Éowyn, how come you here? What madness or devilry is this? Death, death, death! Death take us all!’
and in the next paragraph
‘Death! Ride, ride to ruin and the world’s ending!’
(later in ROTK book 5, chapter 6)
The line "Forth Eorlingas!" is Theoden's (though the Rohirrim also say it en masse), but it's from the chapter about Helm's Deep and the one preceding it. (TTT book 3, chapters 6 & 7).
And, look, Theoden's speech before the Charge of the Rohirrim at Pelennor is a cinematic masterpiece. And Bernard Hill (RIP) is probably the biggest reason why; his delivery is unimpeachable. In a lot of ways, the speech in the movie is better than what's in the books at that moment; but honestly they are trying to do different things.
So technically, the movie version is a bit heightened from the book version, and cobbled together from a few different sources. To put it all together:
Arise, arise, Riders of Théoden!
Verbatim from Theoden's speech in ROTK chapter 5. Correctly placed in the narrative.
Spear shall be shaken, shield shall be splintered,
Also from Theoden's speech in ROTK chapter 5, but not quite verbatim (they added a "shall" in there, presumably for flow...or maybe just Bernard Hill forgot the exact line). Correctly placed in the narrative, though they left out "Fell deeds awake: fire and slaughter!" before this line.
a sword-day, a red day, ere the sun rises!
Verbatim from Theoden's speech in ROTK chapter 5. Correctly placed in the narrative.
Ride now, ride now, ride!
Also from Theoden's speech in ROTK chapter 5, with an extra "Ride!" thrown in. Correctly placed in the narrative.
Ride for ruin and the world's ending!
Death! Death! Death!
Transplanted from Eomer's speech in ROTK chapter 6, with "to" replaced with "for," the order of the lines reversed, and the mood changed from despair to defiance.
Forth Eorlingas!
Right character, wrong moment. Transplanted from Theoden's speech in TTT chapter 7 at Helm's Deep, but in the same vein as Theoden's shout of "Up Eorlingas!" from the beginning of ROTK chapter 6.
So Kelly, from the original tweet, probably overheard her husband reading Chapter 5 of ROTK, and her brain filled in the movie version of the quote while she was writing the tweet.
When I have kids, one of the greatest challenges I'm anticipating is going to be keeping all the voices of the dwarves in the Hobbit diverse yet consistent.
It wasn't lord of the rings, because the kid wasn't into that level of vocabulary yet. It was Harry Potter. But apparently, me booming out every line of Hagrid's in a faux Scottish accent was "going to keep the kid and the entire neighborhood up all night, you damn nerd."
How old are these kids? Because those books get impressively eloquent as they advance, not to mention disturbing:
A creature of an older world maybe it was, whose kind, lingering in forgotten mountains cold beneath the Moon, outstayed their day, and in hideous eyrie bred this last untimely brood, apt to evil. And the Dark Lord took it, and nursed it with fell meats until it grew beyond the measure of all other things that fly; and he gave it to his servant to be his steed.
Are kids gonna love that or wonder what the hell he's talking about? I can't decide.
I just finished reading the LotR series with my now-11 yo son (we started a few years ago) and there were definitely plenty of sections like this that I converted to more understandable terms and concepts on the fly. Sure my son may not have heard this exact line, but he definitely got the idea that Shelob was old, bad, and Sauron had encouraged get 8 in exchange for a scary guard in the tunnels into Mordor the fell beasts were scary.
Edit: apparently my son may or may not have gotten an entirely different version of the book told to him!
man my OED app has been giving bullshit words of the day like "frenzy" "prestige" "vacant" and stuff ive known since i was a kid, and here on twitter you get gems like "verve" which is an actual WotD
Wow I feel like almost all adult native speakers will have heard of a feeding frenzy, a vacant lot and a prestigious award. Those look like good words of the day for speakers of English as a second language of 2-5 years maybe.
right? I had to check myself: "am i that pretentious that I use this many words average people don't know?"and there's no good way to ask that question without sounding even more pretentious