Mythology and the Real World Collide
Like many of his fellow Victorian poets, Tennyson was interested in mythology, and in English and Norse mythology in particular. Everyone in the nineteenth century was interested in origins and history (this was, after all, the time when Darwin first published Origin of Species). Biologists were studying evolution, geologists like Charles Lyell were studying the history of the earth, and poets and other writers were studying the history of mythology and literature. So it's no wonder that many of Tennyson's poems touch on some of these ideas: "The Lady of Shalott," for example, is about one of King Arthur's knights. "Ulysses" is about the Greek hero Odysseus.
"The Splendour Falls" might not be about a famous mythological figure, but mythology and legend still manage to creep in: the speaker is watching a sunset (ho-hum!) and then the sound of a bugle horn in the distance makes him suddenly imagine that the sound is coming from Elfland, the home of all elves and fairies. So long real world, hello myth!