Here comes the criticism. There was quite a bit to comment upon in your sheet.

[quote][b]Who and what are you? From where do you come, to where are you going? Tell us about yourself.[/b]
I was the human Jerek Kahn, but now I am the Sith Darth Scelus. I hail from the desert world of Tatooine. I plan to travel to the seat of power over the Empire.[/quote]
No Darths. No exceptions. At the very most, Sith Lord, but probably not even that. Rising in rank in the Sith Empire isn’t easy and the ones with power do not promote anyone without good reason. The sort of descriptions you have provided below, indicate that his master was probably barely a Sith Lord. Most likely a lesser Sith Serving Darth Arctis, the Dark Council Member in charge of the Sphere of Ancient Knowledge and thus in charge of the Imperial Reclamation Service.

You describe below that your character was sent to the Hâsk & Darth Nyiss. That would indicate that he was not of any noticable stature, at most just past apprenticehood or maybe even still an apprentice. Darth Nyiss, the Sith in charge of the Hâsk, could easily be one to whom someone would be sent to be taught basics of arts such as Sith Alchemy. 
[quote][b]Few men truly serve more than one cause. Sure, you might pay lip service to many and support others, but which cause do you serve most? The Galactic Republic and the Jedi order, or the glorious Sith Empire? If you have other causes you pay some service to, you may mention them as well.[/b]
The Sith Empire[/quote]
Does he truly just serve the Empire? Is he purely idealistic and utterly loyal to the Dark Council and the Empire?
[quote][b]Eyes grow weak with age. They cannot see you clearly. Would you kindly describe what you look like and the sort of clothes you wear? Do not leave out anything you think would be relevant. Are you young? Old? Human? Alien? Male? Female? Scarred? Deformed? Yellow? Blue? Green? Red?[/b]
Jerek is a tall and lanky man, reaching up to 6’ even and weighing 170 lbs. His jet black graying, untidy hair reaches down to brush his shoulders lightly. His once olive-colored skin obtained from years of living on Tatooine is now near the shade of porcelain from years of using the Force in nefarious ways and for his own personal gain. His eyes have turned a dark yellow with tinges of red here and there. Jerek has very angular features, a pointed chin, a long pointed nose with wide nostrils, and a set of full lips that some would say make him look a bit feminine. He uses his long, nimble fingers and long legs to great success in the Niman style of saber combat. Jerek is scarred across his back and arms from years of harsh training.[/quote]
How old do you imagine his being?
His history indicates he’s fairly young, barely been with his master a few years.
His skills contradict themselves by having great combat (especially tactical) skills indicating that he’s pretty grown up, yet he has zero other skills, so in that he’s a child.
His appearance in the portrait below shows no sign of aging, but grown up.
Decide on one thing, would you?

[hider=images]Portrait:
[IMG=http://i243.photobucket.com/albums/ff9/nacron/Baron_by_DavidRapozaA_zps8e814f_zps102b5e88.jpg]

Sith Attire:
[IMG=http://i243.photobucket.com/albums/ff9/nacron/Sith_Lord_by_overdrivezeroo_zpsdb9897b3.jpg]

Mask: 
[IMG=http://i243.photobucket.com/albums/ff9/nacron/a7c0aa54-fe22-4b31-91bc-a19a813a6c17_zpsf18fa84a.jpg][/hider]
#1: Not bad. Own creation or something you dug up on the internet?
#2: Fairly classic Sith Attire. Acceptable. 
#3: If he’s imitating tusken raiders, then the mask will resemble theirs, not a mouthless version of the Guy Fawkes mask.

[quote][b]Now you have described somewhat about yourself, but you have not covered why you fight. Is it out of idealistic belief, or is it something else?[/b]
I fight to prove I am powerful.[/quote]
Please provide more of a reason. Remember, this RP is advanced. He shows some combat skills, but no power beyond that of a strangely tactical brute is described. Where Sith Politics go, this leaves him terribly weak and easily outmaneuvered. 
[quote][b]Surely you are not like everyone else. What sort of skills make you stand out from the rest? Do they stand out because you excel at them, or because you fail miserably?[/b]
I am a clever tactician and can almost see the flow of battle as if it were a physical thing. I am a dexterous fighter, but not too strong; I prefer to dodge around my enemies' blows and time my attacks around them at their vital spots as opposed to overwhelming them with sheer strength.

I am unable to pilot a ship myself. I cannot withstand a strong blow during combat straight out, as I am not too strong.[/quote]
So, you’re a warrior. Your only skills are combat-based. You have no skills outside combat. You do realize this RP isn’t combat-oriented, right?
[quote][b]You have not been in touch with the rest so far. How do you relate to them? Are there any friends of yours among them? any enemies? Or perhaps you even have family therein? Do not hesitate here because you do not see yourself mentioned in other players' character sheets. They may simply have forgotten to write you up. Correct that mistake, would you?[/b]
I have no friends. I manipulate and, at best, tolerate those I see as useful or formidable.[/quote]
Try again.
[quote][b]The past makes you who you are. What sort of events made you the one you are? Do some events mean more to you than others? Did you have a good or a bad life?[/b]
My birth family was slaughtered by the Tusken Raiders of Tatooine when I was but a babe, but then the Tuskens took me in and raised me as their own, and it is because of this that I wear a mask to conceal my face. They taught me their ways of fighting and of life. I still hold on to those ways. My Tusken tribe was taken from me as well, though, when a band of mercenaries came through and killed them for their slaughter of my previous home. Thinking I was just another Tusken child, the mercenaries turned their guns on me as well. Then, a hooded figure came out of the desert and saved me with flashes of lightning from his hands and a red blade he whirled around himself. He killed the mercenaries and then took me in and soon began to train me int he ways of the Dark Side of the Force, telling me that I would be able to protect what is important to me with it. During my training, we traveled from planet to planet, searching for powerful tombs and artifacts belonging to the ancient Sith. It was years later that I found out he hired the mercenaries to kill my tribe just to get to me. That was when I took up my own blade and killed him. Since then, I found the Empire and joined its ranks.[/quote]
This history is… lacking. Remember, this RP is advanced.

What would compel any Sith to jump in and save someone dressed like a Tusken raider? Remember, the Sith Empire view creatures like Tusken Raiders and Jawas as beasts. The thought of such being Force-Sensitive is blasphemy to most Sith. Jedi are the ones who might come in to save innocents. Except most Jedi have been busy for the past twenty-eight years fighting a war.

Killing your master, especially if there’s proof you did it, is a crime in the Sith Empire. As for “finding the empire” after killing your master… That is utterly without plausibility. The Empire is all over the galaxy and has been so for decades. Yes, they do have a presence on tatooine, so even an idiot would’ve known of them unless he happened to be stuck as a lone hermit on a single planet with no outside contact.
[quote][b]No good character is without his or her flaws. Nobody’s perfect. The heroes, be they good or evil, tend to have more blatant flaws than the tagalongs. What be thine flaws?[/b]
Answer[/quote]
This bit is a key point. Fill it out. The more flaws, the better.
[quote][b]No matter if you are a Jedi or a Sith, you probably made a lightsaber once upon a time. What does it look like?[/b]
I wield a curved-hilt lightsaber with a dark yellow blade.[/quote]
Most Sith use artificial red crystals because they’re more powerful. Why would he use anything else?
[quote][b]If you wield a Lightsaber, which of the Seven combat forms has your order trained you in?[/b]
I use Niman style.[/quote]
Why?
[quote][b]What sort of person are you? Do you use the Force for combat, or do you prefer to wield its more mystical branches? Is there something you excel at above all other things? Is there something you are hopelessly bad at? What limitations do you have?[/b]
I am also powerful in the ways of the Dark Side of the Force; my specific Force abilities are: Force Horror, Force Wind, Force Scream, Force Concealment. I can very slightly augment my movements with the Force, but it is not one of my strengths. I prefer the more mystical branches of the Dark Side, such as Force Horror, as stated previously.[/quote]
Sundered Echo will cover this in more detail later, when he looks it over.
[quote][b]Don’t forget that this is in the middle of a war. Anyone of truly great skill would have been at or near the front lines.[/b]
I have seen battle, yes, but I am usually sent on archaeological digs because of my background and experience with the artifacts of old.[/quote]
Where is this experience? Nothing in the background says anything on skill with artifacts. All it describes is a pair of grave-robbers who happen to be Force-Sensitive. And the Skillset you provided is purely combat. No reason whatsoever to be away from the forces striking at the worlds the Republic were treaty-bound to abandon.
He would be SENT to combat if he was good at it. It is a time of war.

[quote][b]You are a Sith. How come you arrived now, several days after the others? Why would the powerful Darth Nyiss let you in at such a late time?[/b]
I was on a rather important archaeological dig when the assault was occurring. Upon my return, I was sent here.[/quote]
Why, make up your mind. You are either an Archaeologist or a warrior. You can’t be specialized in two such disparate skillsets. If you’re an archaeologist, you would not have any logical reason to be a good warrior or tactician. You would certainly not have large-scale military experience. You get that from being on or near the front lines commanding/fighting. Not while grave-robbing.

[quote][b]Is there anything else you would like to mention about yourself? You know there is something. Don't be shy.[/b]
Nothing.
[/quote]