Well, here's what I pick out of it.
[quote]Her friendly expression became brooding, adept[/quote].
...Was it brooding or adept? When you use two adjectives each other like that and they're unrelated, you generally use 'and' after the comma.
[quote]Yuxi did not grow angry, only annoyed, "[/quote]
The comma should be a period.
[quote]nd you have such astonishing vengeance.[/quote]
Vengeance isn't really a trait, it's more of an action. :3
[quote]Not only this, but the Serpent is too wrathful and less intelligent than the dragon. [/quote]
This is a little awkward sounding with the 'too wrathful' and 'less intelligent' right next to each other.
[quote]They feel rage above all other feelings, and their low wit will not allow them to think about anything else.[/quote]
'Low wit' is kind of weak. There are a lot of great ways to say something is unintelligent.
It's in part one, but 'endowed' isn't really the right word for that situation. :3
Also, it would be a good idea to establish a setting. For all we know this whole discussion is taking place in a Times Square burger king. A setting is key for helping people visualize. Part of me wants to say you should also add more physical description to the dialogue, but if you're trying to keep this prelude almost sort of like an old fable, I can understand just having straight dialogue. It's ultimately what you're going for; no description makes it feel like a Genesis-esque 'this is the tale of how it started', whereas description makes it more vivid and tangible. It's up to you. ^^