I recently played the Force Unleashed Ultimate Sith Edition and came up with this idea. Serious AU stuff in here. And there is NO Grievous in here.
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For every bright light, there is a dark shadow.
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Obi-Wan Kenobi noticed his wrist communicator flashing as he was talking to Anakin. He activated it, the figure before him dodging blaster shots and swinging his lightsaber in defensive motions. "General Kenobi?!" the man said in a panicked tone. "Yes Master Barrek?" Obi-Wan asked.

"Need immediate evac from Planet Hypori! Our forces are totally destroyed! Not many of us left!" Barrek said as he dodged left and right on the small hologram.

"There's some new Sith general! He's unstoppable! You must hurry, Lord Starkiller is hunting us! GWAAAAH!" he yelled as the hologram went dark. "Master Barrek?! Come in Master Barrek!" Kenobi started yelling in alarm.
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Hypori

Kenobi's communication was cut off as a large black boot smashed the communicator.

The droids continued firing at the downed Republic assault ships in an effort to wipe out the surviving Jedi. A hand coated in a black glove with claw-like attachments on the fingers raised up into the air, and the droids ceased fire.
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Inside the remains of their starship, the six surviving Jedi were trying to catch their breath. Ki-Adi-Mundi, Shaak Ti, Aayla Secura, K'Kruhk, Tarr Seirr, and Sha'a Gi were the only ones left, and they weren't sure they would get out of there alive.

"He is unstoppable," K'Kruhk panted, his lightsaber in his hand. Shaak Ti nodded and said "We've never before been outmaneuvered by droids. His strategy is flawless."

"We're doomed," Gi said in a depressed tone. "Enough of that talk! Focus, we are Jedi!" Ki-Adi-Mundi said in an encouraging tone. Everyone nodded their heads in approval. But then Starkiller made his announcement.

"JEDI SCUM," his voice reverberated through the air. His voice was deep and inhuman, and it struck fear in many people. All the Jedi immediately jumped to attention, heading for all the openings to keep an eye out for the new Sith Lord.

"YOU ARE SURROUNDED, YOUR ARMY DECIMATED IN MINUTES. MAKE PEACE WITH THE FORCE NOW, FOR THIS IS YOUR FINAL STAND. BUT I AM NOT COMPLETELY MERCILESS. I WILL GRANT YOU A WARRIOR'S DEATH. PREPARE TO DIE!" Starkiller announced. Then everyone got at attention, for they knew that this Sith Lord was powerful.

For a few seconds everything was dead silent. Then they heard the breathing.

It was a chilling sound. The oxygen going in, and the carbon dioxide coming out, slowly. The breathing seemed to echo throughout the remains of the ship. HOOOO CUHHH, HOOOO CUHHH.

"That noise..." Shaak Ti said, fear in her voice. "He's coming to fight us, alone," Ki-Adi-Mundi said in a strangely calm tone.

Gi was the first to crack, as he activated his lightsaber and ran outside, screaming a battle cry. "NO!" Ki-Adi-Mundi yelled, but it was too late. The padawan ran out into the open, only to be hit with Force Lightning.

He cried in pain until a dark form fell from above and stabbed a red lightsaber blade through the padawan's back. For the first time, Ki-Adi-Mundi got a good look at Lord Starkiller.

He wore a black suit that covered almost every part of his body. There appeared to be some kind of gray harness around his waist, and a belt. There were multiple lightsabers on the belt, no doubt taken from Jedi he had slain. On his shoulders, near his head, were some kind of bronze platings, and next to them were real silver shoulder pads. His feet had heavy boots on them, and his legs were wrapped in some kind of cloth that moved as he walked. His hands had many prosthetics on it, with sharp claw-like structures on the fingers. But the scariest part was the helmet. It looked similar to an ubese mask, with the visor and mouthpiece. But the visor was T-shaped, like a mandalorian helmet. On the sides of the helmet were outward slanting metal plates that appeared to protect the neck.

Starkiller chuckled in that horrible, deep voice before leaping back onto the top of the ship, away from the Jedi. His breathing could still be heard by the Jedi.

They all ignited their lightsabers and began to back away slowly from the large opening Starkiller was just in. Thanks to the shadows, no one would look up and see Starkiller studying his prey.

He then released the magnetic grip on his boots that held him to the ceiling and activated his red lightsaber as he fell to the ground. When he landed, he released a Force wave that sent the Jedi flying. They quickly recovered and charged at the Sith. He held his lightsaber in reverse-style Shien grip, something only the best lightsaber wielders could use effectively.

They attacked him as one body, but he leaped, sliced, and blocked his way through their forces. He then jumped into the air and straightened out his body, knocking everyone back with a Force Repulse except for K'Kruhk, who charged forward.

Starkiller took one swing, severing the top of K'Kruhk's lightsaber. Starkiller then threw his clawed hand forward, slicing into K'Kruhk's body. Starkiller retracted his hand, which was now covered in blood.

Shaak Ti gasped in horror and sent a large pile of junk at Starkiller, who knocked them aside with a wave of his hand. Tarr Seirr ran forward, his lightsaber held high. Starkiller smiled in his helmet as he threw his lightsaber at Tarr Seirr, which impaled itself in his heart. His face took on a surprised look as the life left his eyes.

Aayla Secura charged forward as well, but Starkiller was ready. He used the Force to grab Tarr Seirr's body and smash it into Aayla, sending both flying into the ceiling. His lightsaber fell from Tarr Seirr's body and he grabbed it out of the air, igniting it and turning on the remaining two Jedi.

They attacked him together, forcing him to swing left and right to block their attacks. Starkiller then grabbed Ki-Adi-Mundi's arm in his right hand while blocking Shaak Ti's blow with his left (where his lightsaber was). He then pulled the Force into a kick he used on Ki-Adi-Mundi, knocking him into a junk pile and causing him to lose his lightsaber.

Starkiller then went full power on Shaak Ti, swinging like a madman. Shaak Ti, while an excellent Jedi, was losing this fight. Starkiller was too fast and too strong. "Say goodbye, Jedi," he hissed as he jabbed his lightsaber at her heart. She used the force to block it, but the power of the blow sent her crashing into a wall, unconscious.

Starkiller sensed Ki-Adi-Mundi behind him and turned. The Jedi tried to call his lightsaber to him, but Starkiller smashed it with one stomp of his boot. Starkiller raised his lightsaber up, preparing to attack.

Ki-Adi-Mundi, thinking fast, Force pulled one of Starkiller's lightsabers to himself and ignited it, a green blade coming out. "Any last words, Jedi?" Starkiller asked as he prepared to lunge.

When Ki-Adi said nothing, Starkiller said "Guess not" and lunged. Ki-Adi lunged as well, knowing that he would not survive the oncoming battle.
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Okay, this is my first Star Wars fanfic, so don't burn me. Starkiller's outfit is the one that appears on Hoth in the Ultimate Sith Edition. It looks kinda like Darth Vader's. Okay, I'll post soon, and you guys review in the meantime.