Sebastian's personal history has yet to be disclosed, but as leader of the Red Blade he is legendary. For untold millennia Sebastian and his crew were the leaders of Khera's military. They were the most skilled and bloodthirsty warriors ever seen. They ravaged billions, conquered or destroyed countless races, and struck terror in the hearts of whole galaxies. Even noteworthy warlords like Majestic trained under Sebastian as part of his crew. The Kherans in general were a warlike race, but even among them, Sebastian and the Red Blade were considered exceptionally violent. They were the best though, and with Sebastian leading the charge, no enemy could challenge Khera.
The Red Blade lead war and kept Khera safe until they met their match 10,000 years ago. An alien invasion force from the Bleed attacked Khera and was almost victorious in their efforts. All of Khera was at war and things looked grim until Sebastian rallied his troops and fought the invaders back into the Bleed. They couldn't beat them, but they were able to keep them in the Bleed, but to make sure they couldn't get out, one ship had to go in with them and hold them off until the rip in the Bleed was closed permanently. Of course Sebastian chose to take his people and go. Thanks to him, Khera was safe. No one shed a tear at the loss of Khera's cruel protectors, but thought lost forever, they became legends.
Kherans are hard to kill however, and Sebastian would eventually find his way back to this dimension. When the world ended a few nations were able to protect themselves. One of these was Finland and it was able to stay safe by putting up a large force field. Unfortunately the generator powering the force field somehow ripped a hole in the Bleed and Sebastian's ship fell out into Finland. Sebastian got to work quickly and hid his people's presence from the rest of the world while he developed his plan. Using some technology he had access to he planned to shut down Earth's magnetic field, exposing it to the sun so it would be wiped clean (except for Finland, protected by the force field) and then bring it back up so he could start terraforming it to suit his liking, creating a New Khera.
Stormwatch found out what they were attempting and tried to stop them. Jackson King even went as far as trying to ram The Red Blade with Skywatch, his own orbiting installation. This was successful only in damaging Sebastian's ship and upsetting him. Sebastian reappeared shortly after that when the Carrier was mysteriously repowered and Jack Hawksmoor had united many of Earth's heroes to offer them the opportunity to come aboard the Carrier to search the stars for something to fix the planet. Sebastian demanded that Jackson hand over the Carrier as a replacement for the damaged Red Blade and was ready to take on the united Wildstorm heroes with his crew at his side. The heroes that would remain on Earth fought the Kherans to cover the Carrier's departure but were outmatched. Sebastian's battle was interrupted when a fleet of ships from Sliding Albion attacked. This only slowed Sebastian momentarily though. He boarded their command ship, killed their leader and sent them running so he could get back to his fight against the Wildcats.
In the battle against Earth's remaining heroes, Sebastian lived up to his title of Warlord and defeated all challengers, including Black Halo,Fuji, Midnighter, Apollo and even Majestic. Sebastian was only defeated when Spartan had Sebastian's terraforming towers teleported into his ship. When Sebastian went to salvage his ship, some of the heroes sacrificed themselves to keep him on it while they crashed it into multiple Doors in the hopes of trapping Sebastian in the Bleed again. Though it cost them their lives, their plan seemed successful and Sebastian was once more exiled to the Bleed.