A sunset is what occurs as the sun passes below the horizon in the transition from day to night. Because of the way light is refracted from the low angle, they often appear a brilliant red or orange colour. Note that 'sun set' is not the same thing - 'sunset' is a noun, whereas 'sun set' is a noun and a verb - telling us what the sun did (it set).
E.g.
Here are some more examples using the noun.
There is a beautiful sunset happening right now.
It is difficult to describe the beauty of a sunset.
The adventurous man trotted his horse off to the west into the sunset.
The Grand Canyon at sunset is truly a sight to behold.
Example sentence - She was taught to shoot the arrow towards the target.
She was enamoured of her boyfriend as she could not stop thinking about him.
I was struck by the pulchritude of the sunset over the ocean.
Yes, a 5-syllable sentence for "sunset" could be: "The sun sets low."
A gentle caress from his wife restored faith in him.
No, "The brilliant sunset over the desert" is a sentence fragment because it lacks a subject and a verb. A complete sentence needs both a subject (who or what the sentence is about) and a predicate (what the subject is doing or what is happening).
A geographer could tell you what kind of rock is in that formation; all I can do is tell you I love the shadow it casts at sunset.
The photograph would remind us all about the prom.