Setting out to create a bit of a dark rpg, I decided to try to create a game in which characters are powerful but at a price. I wanted this price to be a part or all of their humanity/soul or persona.
I was very inspired by "the dark side of the force" in Star Wars.
I didn't sit down and write an example of play as a start, but I had a pretty clear idea of what I wanted from the beginning, and have as I have been designing the mechanics had a chance to rethink (in a good way, I think) my initial idea many times.
I would like to post an example of play now and then post the rules that I have so far.
My questions to you are the following:
Could I just as well use another system for this(such as Sorcerer(that I haven't played))?
Does the parts of the system that I have posted support my idea(the idea in the play example), and of course general feed back is appreciated.

Bit on setting and the classic questions:
Play takes place in crusader era palestine and surrounding areas. The main adversaries that the PCs fight against are aliens who are (secretly) doing research (with disregard for scientific ethics) on humans. To battle the aliens and other obstacles, the PCs will have to use Power, an incredible ability that allows them to expand boundaries of human capability.
But Power corrupts. Thus as the PCs try to(they can suceed) save the ones they love, they (almost certainly) sacrifice themselves or at least big parts of themselves (and people they love).
The PCs can themselves be experiments that the aliens are overseeing, and therefore they have Power, but I am not 100% this is the way I want to go.
Now for the example of play:
Player: I want to spend a Story Point to set up a scene where I help a friend build the wooden framework for another level on his house.
GM: Ok. Who do you want in the scene.
Player: Me, my friend Walid and one of his friends Abdallah
GM: Ok. Blow us away.
Player:
As we are working on the roof of his (now) one story house in the hot sun, Walid drops a wooden beam on my foot. With the pain a hatred fills my heart and mind.
He of course opens his mouth to apologize, but I have already placed my hands firmly round his throat squeezing hard and I violently bang him up against one of the support beams, that we put up earlier.
I scream at him "you son of a bitch! Is this how you repay me?" while Abdallah stands next to me pleading me to stop, desperately yelling "what are you doing? It was an accident for the love of God".
Walids eyes full of fear and confusion bring me back "closer to reality". I let him go, looking at my hands that are still twitching in a choking motion, as though they did not wish to stop. Walid is sitting at the base of the beam, gasping for breath. I hiss that he is "worthless" and jump of the roof. As I walk down the street Abdallah and a few neighbors look at my back confused and frightened. Looking mostly at the ground I run down the street crying and thinking "what have I done", my rage turning into self-hatred.
GM: Very nice. I suppose you are acting out your Passion Corruption which is Flame .
Player: Right

Tomorrow I will post the mechanics. Feel free to give me any input already.

I have decided to let Power be the result of alien experimentation. PCs are either the direct experiments or descendants.
Games can be set in early period where players possibly do not know what they are or late period where PCs are part of network trying to bring the aliens down, and possibly having other agendas as well.

Ok now for the mechanics. Please just skim past the first 15 or so lines as they are not very interesting and very very rough(the rest being just very rough:) ), but I thought they had to be included in the presentation. It gets more interesting once you hit the Power title, the Corruption title and the Plot title.

Resolution
Take relevant attribute. Depending on skill level you add or subtract a randomized number by throwing dice:

Unskilled : subtract 1d3-1

Skilled : subtract 1d3 add 1d3

Master : add 1d3-1

Your Resolution Result(RR) is compared to a Target or other characters RRs.

4 Attributes
Attributes range from -3 to 5, 0 being average human. Power starts at 2 for PC's. Body, Mind and Spirit have to be set by the player to equal 5.

Body
How good you are at using your body. An old man who is a master of martial arts could have a higher body rating than a bodybuilder.


Mind
How intelligent and wise you are.


Spirit
Force of personality, psyche and soul. Your best defence against Corruption from use of Power is to have a high Spirit.


Power
Power is a feeling you always had that the world would bend to your wishes if they were strong enough. Power can let you do unimaginable things by sheer force of will. But at a cost. While you expand the boundaries of what human body and mind can achieve Power burns a hole in your soul and tears your personality to shreds.
You can only take as many points of Corruption at a time as your Power. When you increase Power you become a bit better at handling the Corruption(you move border numbers for levels of Corruption), but not nearly as much as when you increase Spirit.

When using power it corrupts. Depending on what kind of results you want from your use of Power it scars your soul and personality in different ways.

Using Power
There are 3 main ways of using Power:

Body Boost
You can use Power to boost your physical abilities. For each point of Corruption you increase your Body by 2.

Magika
Magika covers things like telekinesis, pyrokinesis, healing(these are not spheres of magic or disciplines or something that you can train in as such, they are just different ways of using Power) and other things that are done "outside your anatomy". Human anatomy basically allows for you to jump over a 50 ft. building, you just need really strong leg muscles, thus jumping over a building is a Body Boost not Magika.
Magika does not cover however the manipulation of other peoples minds. This is covered by Manipulation.
For each point of Corruption taken you increase your Mind by 2 for Resolutions covering Magika. Magika will usually be against a Target set by GM. You roll dice as if you were Skilled.

Example Targets
Healing one category of Injury: Target 6


Manipulation
Mind-tricks, illusions and even telepathy are kinds of Manipulation. For every point of Hallucination & Delusion Corruption you take you add 2 to your Spirit for Resolutions of Manipulation. Manipulation will often be against other characters Spirit RRs. You roll dice as if Skilled (and so do characters you are trying to deceive).

Corruption
3 kinds of Corruption

Passion
Humans are emotional beings. When you use Power in situations where you can be described as highly emotional you take Corruption in Passion. Because of the nature of Power, when using it for combat or other kinds of violence you always take Corruption in Passion. Another example could be healing your best friend, girlfriend or family members.
If you have a high Passion Corruption your emotions are strong. So strong that they lead to tragedy.

Egomania
The use of Power can quickly go to one's head.
Examples are many: healing self and basically all Magika and Body Boosts where Corruption is not taken in Passion.
If you have a high Egomania Corruption, well you become an Egomaniac. You see yourself as more important and valuabe than others. If others begin to shun you because you are basically a pain in the ass, it fits perfectly with your feelings that they do not appreciate you and what you are willing to do for them. At very high levels your egomania can begin to turn into complete lack of sympathy for other beings, mixed with Passion Corruption and a lot of Power this is an extremely volatile cocktail.

Hallucination & Delusion(H&D)
When you do Manipulation on others it tears your own ability to perceive the world correctly. This trick of bad karma manifests itself in Hallucination & Dellusion Corruption.
H&D covers visual and audio hallucinations. But this Corruption also covers the occasional dellusion or perceiving things differently than they may actually be "Didn't that barkeep just put poison in my ale?" or "My old friend is out to hurt me".

Levels of Corruption
There are 4 levels of corruption, Ember, Flame, Inferno and Ash. Ember is when Corruption is equal to or under Spirit. Flame is when it is below or equal Spirit + Power. Inferno is when it is below or equal to Spirit x 2 + Power. Ash is when it increases above this.

When it is Ember it does not seriously affect the character. When it is Flame it affects the character, and when it is Inferno it greatly affects the character. The player is rewarded if he plays the character as doing things that show how affected the character is. A character with an Inferno Passion Corruption might kill even unarmed enemies without hesitation, or he might shower a girl he loves with gifts even though they have just met (and she will be a bit freaked out by his obvious intense feelings). He might even go and rape someone because of the immense feelings of lust going through his body.
The player is rewarded with story points when he plays out the ways in which the corruption affects him. Corruption is always bad, in the above examples the gift-showering should always lead to tragedy for the character (and/or surroundings) in some way.
When a Corruption becomes Ash the PC takes 2d3 in the other two Corruptions, if they become Ash as well, the PC's personality, psyche and soul is burned out and he dies. If one or more types of Corruption do not become Ash, the ones that do are "re-ignited" and start at Power+1d3.
The PC is aware that when he uses Power it effects him negatively. This is in a way reminiscent to drug addicts knowing that drugs don't really help them, the big difference being that Power can actually solve problems and that Power isn't really addictive as such.

Lowering Corruption
During creation every player chooses 2 traits that define his character. These are personality defining traits and often positive. Examples include pacifist, religious, helpful but also drunkard and maybe bad-tempered. When the PC acts according to his traits he can lower Corruption by 1 point. Acting according to a trait could be helping a friend build his house with no reward for most of the day, or going to the pub drinking and basically doing what a drunkard does for most of the day. It is not enough to attend mass on Sunday if you have "christian" as a trait.
A trait can be changed at the cost of 1 SP but something must happen to show that the character has changed.

Plot
2 arenas of play
Environment
Environment is everything around the PCs. This is usually controlled by the GM in a "traditional rpg" way and interacted with by the PCs. The adventure unfolds in the Environment.

Personal
The PC and everything else controlled by the player. Because of the Hooks and Personal Scene mechanic the Personal sometimes involves parts of the Environment.

Story Points
Story points resemble the PC's importance to the story. It represents his ability to develop as a character as it is used to increase attribute's and skills. A PC starts play with 3 SP.

1 SP can be used to say i would like "bla bla" to happen in the Environment in this gaming session. GM can turn this down when put forth and the SP goes back, if he does not he is not obliged to let "bla bla" happen but if he doesn't the player gets 2 SPs back at the end of the session.

Personal Scene (the example of play is a Personal Scene)
1 SP can be used to take control of a scene or set up a scene in which the player has a larger control of drama because he controls the action of his own character and Environment. Thus he can be sure that he gets a chance to act out his Corruption(or something else he wishes to convey). The GM has to accept the NPCs in the scene and the GM can veto the actions in the scene or if he really feels he has to, take back the control at any point, but then the PC gets back his SP(unless they agree that the player has acted out what he wanted to/needed).

SPs also work as "hero points". You can spend 1 SP to increase or decrease a resulotion result by one.

Earning Story Points
General good acting on behalf of the player, specifically acting out Corruption, in a scene set up and run by the player at a cost of 1 SP or in the GM run scenes.

Goals
GM sets out goals during play that you can stake SPs on. If you complete your goal you get double the amount of story points in return. A Goal is thus a tool the GM uses to make the players interested(mechanically) in the adventure.
Players can also propose Goals for themselves and/or other players that the GM can accept or reject.

Hooks
Hooks are individuals or groups that the PC has a connection with. Often individuals that he has a strong emotional connection with. Depending on how general they are a PC should start play with at least 3 Hooks, at least 2 of them being individuals.
Instead of (or in addition to) staking Story Points on a Goal you can stake your Hook(partly or fully). The GM will then use your Hook dramatically to "hook" your character to the adventure.
If you suceed in doing what the Hook "asks of you" you gain 3 SP for a partial stake and 6 SP for a full stake, and your relationship possibly improves. If you fail your relationship deters(partly staked) or you can even lose the relationship completely (fully staked).