Adventure, Quest, Folklore
The Return of the King has sword fighting, sorcery, and the Dead rising from their graves to fulfill an ancient pact with the living. Adventure? Absolutely. Plus, the whole Lord of the Rings trilogy is about the destruction of the Ruling Ring of Power, which sounds like a "quest" to us. Call it an adventurous quest. Or a questful adventure.
As for "folklore," Tolkien draws on elements of folklore like ghosts, elves, dwarves, and goblins (a.k.a. orcs) in his depiction of Middle-earth. The Return of the King may not itself be an example of real, old-timey folklore, but it certainly uses aspects of more traditional legends and songs to shape its settings and characters.