S!
S!

-------------

Nevvias wanted to believe the suffocating heat spewing from the wall of flames was the reason these out-of-focus hallucinations were coming back to him. He saw yet another congestion of churning grays forming in a clearing where the fire wasn't already consuming trees and work tents. Retreating civilians were running through the congestion, as if it wasn't there. Nevvias conceded the fact it wasn't. Not yet, anyway. The sphere expanded. The grays sharpened into an angry orange.
Nevvias stumbled backwards. "Behind you!" he shouted.

A stressed fire fighter flinched his head. "There's nothing---"

Plus, some if it is redundant anyhow. I would take out suffocating, because if it's flames we are talking about, people get the idea. Also, heat doesn't soffocate people but smoke does. Heat doesn't spew, it is emmitted, normally. Plus we kind of know that fire is hot, so you could do more with less by mentioning something else example:

MC wanted to believe it was just the lack of oxygen. The flames were burning hot with thick clouds of smoke filling his lungs, and his breathing was now shallow at best; but he couldn't dismiss the slick grey forms that were gaining on him as mere hallucinations. They were more...

I don't know, you get the point. Just don't feel like you have to put everything into the first sentence. Take you're time. This is a very active scene. There are four huge things going on here. The fire, the possible asfixiation, hallucinations, and the aliens. A fifth is also, that this isn't the first time it's happened. That's alot to throw at someone in one sentence. This kind of reads like a climax not an intro.

You might consider taking us further back. It seems to me how they got there, when and how did the fire start might also be interesting things to see, especially since you have a whole novel here to explain.

Anyhow, the rest flows well, and it works. I'm just not sure if this should be the start of your novel. Keep at it If this is just a beginning to an abduction or something, and this HAS to be the beginning for things to flow correctly later, then I'd simplify things. Make this his first encounter, or at least the first one he remembers, and maybe start 10 minutes before this, before the fire, and hopefully when things are kind of quiet. Maybe, he spots something and investigates...you know that kind of thing. Maybe less cliche then that...but you get my drift.

The first sentence sounds forced, like you are trying to wedge your hook in up front.

What is the hallucination? “Congestion” makes me think of many people trying to shove through a tight space – it didn’t work for me to be applied to an image. I think that word choice is my biggest cause for confusion.

I see what you are doing with the phrases “Nevvias conceded the fact it wasn't. Not yet, anyway.” But it pulled me out; sorry, I can’t say why, and I have no alternate suggestions.

“The grays sharpened into an angry orange.” Now this worked for me. From here on, it works.

The name Nevvias sounds made up, and doesn’t fit with the real-world setting.

Nevvias wanted to believe the suffocating heat spewing from the wall of flames was the reason these out-of-focus hallucinations were coming back to him.
*Why would heat cause hallucinations?*

He saw yet another congestion *Where was the first one? Also, conjestion makes me think of stuffy noses and forms no clear picture of anything in my mind.* of churning grays forming in a clearing where the fire wasn't already consuming trees and work tents. Retreating civilians were running through the congestion *again, a weird word* as if it wasn't there. Nevvias conceded *strange wording; maybe you can say, "so it must be" at the end of the previous sentence* the fact it wasn't. Not yet, anyway. The sphere expanded. The grays sharpened into an angry orange.
Nevvias stumbled backward *no "s"*.

"Behind you!" he shouted.

A stressed fire fighter flinched *strange usage of "flinched"* his head. "There's nothing---"

Flames exploded outward, lashing the fire fighter in the face *only the face?*. The force shoved him backwards, and his scream was drowned out by the surrounding panic. *maybe combine the last two sentences like this*