AUTHOR'S NOTE;;
Oh my gosh, it's the first character!! Submitted by 
I'm so so so excited to be doing this. I've known Shadowscale for a long time now, and I'm continually impressed with how far she's come as a dragon.
Also - just a bit of trivia for you kids out there - this dragon was one of the first (if not the first) hybrid OCs ever to be made in the fandom on Instagram! Rina was one of the first people to join the fandom (literally. She read the first book before the rest of the series was released), and as far as I know she and I were part of one of the first RP groups.
So yeah. Show this mother hecker some respect, children.
Edit - I'm going all-out aggressive because this is my friend and I love her regardless. I swear I will be nicer than this.
- Slightly Aggressive Siri Mom.

--

BASIC DETAILS;;
------ Name: Shadowscale.
Seems fair. I've seen too many dragons with NightWing-esque names (even some that weren't NightWings!) whose names are completely unrelated to their personality, and really just sound mystic. Typically, those names will involve words like moon, dark, shadow, death, soul, blood, ice, heart, star or even galaxy. In the canon, maybe two NightWings had celestial-sounding words in their names; the rest of them had plain words like Bigtail and Fierceteeth and Preyhunter. Words that describe the dragon. So Shadowscale seems like a good choice for this character, since the name does match her description and doesn't sound that ominous. Good choice!

------ Age: 36 years.
Finally finally finally a dragon outside the 7-18-year age group. This fandom needs more mature adult OCs (and come to think of it, more mature people, period). I think that this is a good age to work with, as it still leaves you some youth to work with while still giving you enough of a lifespan to add some maturity in.

----- Gender: Female.
Not much to say here. The male to female OC ratio in the fandom is incredibly uneven, but I can't exactly go around blaming you for making a girl dragon.

------ Sexuality: Queer.
Same, buddy. Same. If you're uncertain on what you'd like for her, I'd recommend researching the different sexualities out there, or asking a LGBT+ person to help you (if you need me I'm here). You could even try taking a "what sexuality am I" quiz and trying to click the answers you think your character would pick - this is actually a great way of getting to know your characters! Although I warn you, there are almost no decent quizzes of that sort out there, as most only outline 3-4 sexualities at the most.

------ Tribe: NightWing/SandWing hybrid (NightWing being the dominant tribe).
This combination is a little overused now, I'll admit, but some leeway can be offered, since this dragon was created pretty early in the fandom's development when the idea would've seemed creative at the time. And also, I do understand the sentimentality involved in keeping the qualities of your first OC permanent (however dreadful they may be).

------ Abilities: Fire breathing, teeth and claws, night vision, barbed tail (it's unable to produce a poison strong enough to kill; however, in rare cases, it can cause temporary paralysis), trained fighting skill.
That's a tad many powers for a hybrid, but I suppose one can't really complain about things like teeth and claws and trained skills. Fire breathing in a SandNight is logical, since both parent tribes do breathe fire (thus making the gene way more likely to be passed down to the dragonet). The only thing I'd maybe consider eliminating is the venom - it's highly unlikely that a SandNight would even inherit a barb in the first place, let alone one strong enough to produce a toxin with any strength.

------ Relationship Status: "None (*cough cough* but I'm thinking about making it Coral *cough cough*)."
Please do please do please do please do.

------ Occupation: Assassin (current), leader of the Dark Scales organisation (previous).
That's a pretty big jump. And kinda ironic - Dark Scales, Shadowscale.

--

PHYSICAL DESCRIPTION;;
Dark gray, monotone scale colors. Dark shades being her dominant color and light gray being a secondary color.
Wow wow wow the American spelling of "colour" is messing with my eyes. Squiggly red lines everywhere. But this is a very good colour combination, and perfectly natural for a Pyrrhian. While technicolour OCs aren't necessarily a bad thing, people need to stop and consider that dragons are still very much natural creatures, and would need to be able to blend in with their environment. Being part NightWing, she would naturally have darker scale colours. So yes, this makes sense.

Shadowscale has NightWing star-covered wings which possess a slight blue coloration.
As a NightWing, this does make sense. I'm not sure where you're getting the blue from - if she's part SandWing, wouldn't she be more likely to have traces of red, gold or dirty tan?

Her eyes are a mutant red color -
What?
Red isn't a naturally-occurring eye colour in any Pyrrhian tribe, as far as I'm aware. If she does end up with red eyes, there's probably something wrong with those beautiful luscious glowing orbs (gosh I laughed while writing that). In the very least, she probably wouldn't be able to perceive certain colours (or, depending on how you look at it, she could be able to see an entire new spectrum of colours). I'm afraid my knowledge on the matter is a little rusty, so you may have to Google that to double check. But it'd definitely be something worth researching.

... and scars litter her body from multiple fights.
Yes yes yes yes yes. So many OCs I've seen have been through battle and ordeals, but almost none have scars.

She has three NightWing-like horns on the top of her forehead and end of her tail.
That's an awful lot of horns, but I'll let it pass, since I suppose it could happen.

Along her body is a SandWing crest and the end of her tail is a black scorpion barb.
When combined with the multiple horns and odd-coloured eyes, this seems a little improbable for our dear hybrid child. I suppose she could inherit the crest, but I'm seriously doubting the probability of that barb's existence, since even [ TBN spoilers ] Sunny, who was dominantly SandWing, didn't manage to inherit one from her mother. [ TBN spoilers over. ] The fact that this barb has venom too only adds on to the unlikelihood; so I'm afraid you've fallen short here.

Her build is muscular but she has an average height.
Normally hybrids would be small or stunted in some way, but I'll let this pass. At least she's not one of those "I'm taller than all of you but it doesn't impair me" or "I'm short but still dynamite" OCs.

--

PERSONALITY;;
Shadowscale is very serious as she gets older and becomes very temperamental.
Ooo, character development from the go. I like it, although you may need to elaborate more on why.

She is very loyal and true to her word, valuing honor over all no matter was rewards she is offered. This shows when [ removed because fanfiction spoilers ].
*heavy sighing*
Lightning strikes to the east. Scrolls and quills clatter to the floor in the quiet of Siri Mom's cave. Dragonets are screaming. NightWings are speaking prophecies. All around us, soldiers are having flashbacks: terrible memories of Eragon, of Bella Swan, of -
I'll stop there (before my heart does). But really. Really. This is one of the biggest, most overused hero qualities out there. Yes, we get it - having a character be a good guy is always a satisfying thing. We want to make them more relatable to readers by emphasizing their struggle for the right cause. Adding this quality makes it look like you've tried too hard to make someone good, and as a result created a knight in shining armour instead of a balanced, three-dimensional character. This could, in a way, work well - if you make the character snobbish and arrogant, or painfully loyal to the queen. But from what I'm getting, Shadowscale doesn't seem to be that sort of dragon; if anything, she seems devoted, humble and loyal. Just like any old hero.

However, when tragedy strikes and the loss of a friend and family Shadowscale becomes vengeful and stops at nothing until she kills the dragon who brought the tragedy upon her. [ Kept vague because spoilers ].
I'm sorry to say it, but this is perhaps even worse than the part before this. Suddenly, instead of the perfect character, she's the tragic villain with a terrible past, driven to terrible deeds all with a valid explanation. If her loyalty and sense of honour were really that great, I doubt this dragon would ever have committed an act as fraudulent as murder - if anything, she would've been more determined to preserve her parents' name and maintain peace in her homeland. What you've done here is simply created a "good guy gone bad" scenario. If this dragon killed her family, wouldn't she be scared, if anything?
To give you credit, at least she only killed that one dragon. She could've gone on a mass murder spree like those terribly creepypastas, but you've had enough sense to avoid this. Unfortunately, you've still fallen into one of the most common personality tropes out there.

Shadowscale's heart was numb for quite some time before another picked her back up and restored her to her normal state.
Aaaaaand it's the cherry on top. As if being broken wasn't enough, you've gone and mended your character right up. It's like it never happened; she's come off stronger, if anything.
This is not how the world works. This is how a very common fictional personality works. If her heart had stayed numb, then perhaps you'd have gotten off a bit more lightly; but the loss of her parents and her own killing of another dragon will have had a lasting effect on her. More likely, she'd never recover - losing parents has a drastic effect on any individual. As does killing; she'd likely be very traumatised and have developed several mental issues as a result of her deed. This could go in one of two ways: she could become overly violent and sadistic (the less likely of the pair, and also ironically the most overused), or she could become terrified and unable to so much as look at a weapon without becoming anxious. Killing and death are two very significant events, especially to a conscious and therefore fragile mind; while Pyrrhian dragons may be a bit more resilient than real-world humans, she would probably still be at least a bit more shattered.
Let's talk about this "another". Was this a friend? If so, fantastic! You may change every single thing about her personality except this.
Was this a lover, ex-lover, future lover, crush or any other form of romantic associate? If so, drop it and run. While the idea of a love interest coming and immediately piecing things together may seem tempting, the message it communicates is ghastly. People need to understand that no Prince/Princess/(Insert Gender-Neutral Monarch Term Here) Charming is going to come and fix their entire world. And based on my experiences with you, I feel like you'd be the sort of person who wanted to communicate a message to people.
Having this role be filled in by a friend completely erases this issue, however. It shows that not everyone can get through things alone, and that not every major helping hand has to come from your one true love.
Wow that's a big paragraph for one sentence.

--

BACKSTORY;; Spoiler warning for Rina's fic, The Black Shadow.
In a summary of her life, Shadowscale grew up in a secluded group of mixed dragon tribes who called themselves "The Dark Scales". She lived with two adoptive RainWing parents: Starshine and Sunflare.
An interesting start so far; I like that you've given her a logical excuse for her hybridism. I'd love to hear more about the Dark Scales in the future!

When she became the right age, she was apprenticed to be an assassin, a job she was given by the leader of the Dark Scales, Northstar.
Although the job is a little overused, I can understand why a large group of dragons would need a good military. What I'm most curious about is why their leader would ask for one personally - does this relate to his personality and/or plans in the future?

However, because of an injury her father suffered by an attack of an unknown SeaWing and was left incapable of training Shadowscale, Northstar, who had no children, took Shadowscale under his wing.
I can't really see why a busy leader would take a young dragon/dragonet under his wing - especially if he's had no fathering experience. Unless he had some very close plans for this assassin child of his (as I hinted in the second paragraph), this seems a bit unrealistic.

She soon learned over time that Northstar was becoming unlike his old self and over time, turned corrupt with power.
Power corrupts. It's simple, it's there. Now we're beginning to see Northstar's motives. While the whole "ruling made me evil as opposed to teaching me good leadership lessons and qualities" card is quite overplayed, it does work in this instance.

He attempted to convince his apprentice that she would rule in his place when he died and would rule with a fierce grip using tooth and claw to make the Dark Scales expand and rule the other tribes.
This guy really seems to have it in for the rest of Pyrrhia; but I'm so glad you've given a reason for him to have even looked at her in the first place. It would be nice to hear a bit about his motives (but I'm assuming you have those separately). Just a lil thought there!

--

COMMENTS;;
------ The good...
For a hybrid, I have to say, this dragon is astoundingly balanced! You've created a very unique blend of abilities, and combined her parents' attributes nearly perfectly. You've kept her within a decent age range and you've also given her a decent name (a rare feat among this fandom's NightWing-centric population!).
I also especially like the attention you've payed to Shadowscale's backstory. You've been sure to give Northstar a very clear motive, and the circumstances seem believable enough. Any tragedies in her life you've been sure to account for in her personality; these you've dealt sparingly, something for which I will be eternally grateful. I look forward to reading about her antics in your story!

------ The bad...
I'm sorry. I really am. But your character's personality needs a lot of work.
What I'd like to see is one major, noticeable flaw that doesn't relate to the tragedy in her past. I'd also like to see an explanation for this overly heroic stereotype you've created; who implanted all this honour in her? A parent? An idol? Scrolls? The grizzled old dragon in the cave next door, who tells her all those wild legends?
Self-generated honour is a rare thing indeed, and all but nonexistent. Which means you probably shouldn't dish it out quite so freely into one character without a very good reason or a better set of flaws to balance it out. All I really got out of Shadowscale's personality is a stereotype: valiant hero turned bitter villain turned slightly more angsty hero again. You need to remember that unless the incident with her family happened to her at a very young age, it is highly unlikely that her entire personality will revolve around it. Yes, a large portion of it will be altered, but we should still be able to see some signs of a dragon outside this cold, ruthless (but also honourable) assassin.

------ How you can improve!
This OC is very good when compared with the majority of the rest of the fandom, but I truly believe you have the capacity as a writer to make her better. In order to do this, I'd recommend changing, researching or just giving a bit more thought to two main things:
Her personality. As I've said, it almost perfectly mimics at least two stereotypes. A bit of spicing up around the ruff couldn't hurt. Treat flaws like colour; don't be afraid to experiment!
Her abilities. Does she really need that venom? It's highly unlikely to occur in a SandNight in the first place; and if she's a trained fighter and assassin, will she really need it? As far as I can remember, Deathbringer (for instance) had no venom, but he was more than able to wound Blaze very efficiently in spite of her barbed tail, Glory's venomous fangs and the four remaining dragonets sitting around them. If I were you, I'd maybe consider giving her a signature weapon of some sort instead.

--

FINAL RATING;; 7/10.
Overall, you've created a wonderful OC with a very interesting description. While there are some aspects you can improve on, I greatly enjoyed learning more about this character, and hope to continue doing so in your book.

Happy writing, and keep sending your OCs in!

- Siri Mom.