We need a bit more information - genre (ok, fairly obvious, but you'd be surprised), length and intentions. Are you looking for critiques of just the first 13 lines, or for readers? Is it part of a novel, beginning of a short story etc.

Couple of technical things...your original is in the []

[When it was free of its den from where it had been sleeping for the last two hundred years it breathed deeply, seeming to pull all the air out of the very area and into its great belly.]

When it was free of its den, where it had been sleeping for the last two hundred years, it breathed deeply, seeming to pull all the air out of the very area and into its great belly.

[It gave a loud grumble after that and peered around until its eyes caught site of the six figures]

"Two gargantuan great black claws slammed down into the mud, sending it flying upwards as the huge dragon pulled itself forward, lazily."
Here, we already know he is black, but the precise color of claws isn't relevant.
Gargantuan great is redundant. Don't need the great.
We probably don't need to know "down" into the mud. I think it's obvious.
Same for "upwards".
My biggest problem with this sentence is the end ", lazily". We get this strong mental picture of motion, and then right at the end are told it was supposed to be a lazy action. Might wanna move the "lazily" to in front of "pulled".

"seeming to pull all the air out of the very area and into its great belly."
Here, I think "area" is a little too vague. Are we in a swamp? Why not the "the very swamp".
You can omit the "and into its great belly", since it's obvious.

I think the sentence containing this has a comma splice. You might want to break this up.
"Great black bat-like wings"
Try to come up with a better acjective than "great black bat-like". The wings would stir the air, for one. If the blackness of the creature is so important, perhaps you can find some good way to get the blackness of the entire creature across in an earlier sentence.

"that were covered in black and green robes."
I'd just say "six robed figures." Try to work the color in as another sentence, if necessary.

"almost trembling"
Try to make more of a definite action. Omit the "almost." They SHOULD be trembling! And if not, the dragon should know why and state so.

Your last line validates your earlier ones about knowing how long the dragon slept, and it makes me assume that this is the dragon's POV. Earlier sentence, though, seem to indicate a different observer - one of the robed men. If your dragon is a main character, you might want to use his gender in the above sentences instead of "its" or "it", since you use his gender in the last sentence. (Someone tell me if this is a bad idea, but I think it's good since this isn't an animal - it's a character).

"The dragon slid the last portion of his body"
He is sliding his body. His body isn't sliding his girth.

"as the huge dragon pulled. . ."
As *he* pulled...

"where he had been sleeping for the last two hundred years"
How does the dragon know this? Especially if these men woke him up? Did they have a schedule to awaken him? If so, disregard this comment.

"The moonlight glistened off the dragon's. . .as it moved, . . ."
*His* obsidian scales as *he* moved. Stick with your POV.
I'd add a period between this and the wings spreading. They are two unrelated ideas.

"He gave a loud grumble after that"
I can't imagine a dragon grumbling. How about a snort, or something? Also, no need for the "after that".

". . .of the six robed figures in front of him"
Probably better to say "below him", since he is so massive and would have to look down to see them. Unless they are on a ledge in front of him, looking straight into his eyes.

"He rumbled out, . . ."
No need for the "out"

". . .in front of him".
I'd omit this, because it's a bit of an echo from the end of two sentences ago. Maybe add something about how he feels about their trembling, or noticing their weapons if any