This is a chapter fluff thing... ya... stupid name but man it seems I good Idea after you hear this...
Fluff
Durinbg the Horus hersy, several thousand traitors began to realize that Horus was on the verge of victory and yet, they felt as it they were about to kill their father and great lord. The Swung at Horus himself, but were defeated. They came from one of the Forgotten Legions, and after the battle the remaining 1200 reverters had a choice, join Horus agian and attack Terra, or be anniahalted. THe leader of the revert was Lord Andres Verso, turned to his men and signalled for a teleport. He remained and defied Horus even further by kill several more Space Marines before be felled by Bolter.
They were transported to a nearby vessels who supported them and the left the system for a nearby Imperium planet. The planet was Ventor III, a small planet half the size of Terra. It was an Agriworld, suppling the Imperium with a small sum of Troops and food for the war. When the Space Marines arrived in Orbit they were meet by several destroyers and Cruisers. After surrendering without a fight the planet soon realized that they were traitors who run from Horus madness. Teh Planet allowed them to land on planet and hide til after the defeat of Hours and they tranported them to the high lords of Terra. After explaining they wish to serve the Emperor again and to ameed for the inproper actions. Several High lords demanded they be put to exicution, while other fought to protect them. Finally the new commander of the Space Marines proposed that two hundred of them to be exicuted. They were selected and quickly put to death. The renamed themselves The Alpha Spartans, deciding on it becuase of their new colors. They quickly returned to Ventor III, thaking them for tanking them in during the Hersy. They left, bent on regaining the trust of the Emperor. Four months later they were recalled to Ventor III to resue it from destrution.
When they arrived back in system to find it undersiege by a a large group of Orks. quickly moving to rescuetheir freinds and allies. For twelve years, the battle raged but the planet in the end was lost. Every citizen on planet was evacutated to the several Space Marines ships helping regiments and marines hold for much easier. But the final order came in and the EXTERMINUS was order on Ventor III. few Imperium citizens were killed (All records of the the battle were deleteed by the Spartans as they consider it a major defeat). no for the past Ten thousand years the fleet has expanded to contain three world lost to the exterminus, the chapter and a battlesquadron from the Imperial Navy. All the regiments from the planets are attached to the ships carrying their world population. The Spartans continuely jump to the rescue of small planets and defend them until complete victory. They Crusade around the Imperium looking to give assitance where ever its need. Presently their located near a large planetary system with no Imperium forces nearby to repair and refurbish their ships and weapons.
They exist in secert with few if any Imperial worlds knowing of their existance. Only three Space Marine chapters have ever know of them... two of them have since been destoryed...
Tactics
Even though from a forgotten legion, they seem to speicialize in close combat and urban strategies. Recent battle with Tau and Ork Forces have made them tried and tested. Very few enemy forces have ever engaged the Spartans and survive. They Use the regiments of the lost planets to the advantage of having a regiment assualting with them. Though very few regiments are armoured regiment, many are veteran's of fighting Ork and Eldar. Spartans have an old saying they were traitors and they saw the mistake and corrected themselves before it was to late. And they still correct the Imerpium to this day...
Battlecry:
"With glory we go to battle to take victory, and in death do we take Honour"
Organisation
1st company
Veteran company specializing in rapid entry and quick swiping victories. 9 Tactical squads and 1 command team. 6 Rhinos and 2 Razorbacks attached
3rd Company:
Close combat company, consist of 10 Assualt teams, used against orks.
4th Company:
Terminator Company, used only in space combat and sqecial operations. 5 Terminator Teams and 4 Assualt Terminator teams and 1 Assualt Terminator with a Chaplain.
7th Company:
Devestator Company. 20 Desvator teams with a 5 man Command team and 12 attached Dreadnoughts.
8th company:
Scout company, 18 scout teams, 8 Rhinos and a 5 man command team.
10th Company:
Armoured Company. Consists of four Land Raiders, 3 Baneblades, one Shadowsword, 12 Landspeeders, 10 Predators, 4 Vindactors and 8 Whirlwinds.
Over all fleet strength, 129 battle ready ships, and 300+transport carrying imperial citizens. 23 Imperial Gaurd Regiments defend most of the fleet with 28 regiment assist the Spartans in ground operations.
Overall Chapter strenght presantly:
750 spac marines

Main Commanding teams look like so... Their hands a black and white, white signafying their purity and black to make them remember their terrbile act of hersy...
Every day space Marine
The pics come from Canonness Rory.