This feels like a sentence out of Game of Thrones.
why don't we just "la" in this question?
I'm not a fluent Esperantist, but why does the adjective "rapida" end with an "e" in this scenario?
"Quickly" is not an adjective, it's an adverb. Adverbs usually end in "e".
Shortly after writing the comment I realized that, but I wasn't completely sure about it.
Aren't there any indefinite articles in Esperanto?
No, there aren’t. Just use no article.