Sev was captured and taken to a secret Separatist facility where he was imprisoned, tortured and interrogated for knowledge of the Confederacy operations regarding the Trandoshans and for any knowledge pertaining to General Grievous.
He resisted for several weeks, not attempting to escape, merely examining the routine he would be put through as psychological torture and the route to the torture chambers (he was masked, but could still feel the surface he was walking on and the turns being walked through).
After he felt he knew that section of the facility well enough, he began to "give in", feeding his interrogators false information. However, after several days of this, they began to realise that there were some inconsistencies in what they were being told (Sev had psychological as well as physical torture, he could not properly construct such a deception).
The torture was suspended, merely leaving him in his cell for months on end with no contact with any lifeforms. They then focused on mental torture, showing him holograms depicting his squadmates abandoning him (of course created by the CIS), and his replacement by a more skilled Commando. However, instead of breaking down his mental defences, these images of his comrades served as a rallying call, and he summoned his strength and willpower enough to attempt an escape. The next time a holoprojector was sent into his cell, he smashed it open and used it to create a small explosive device, which he used the next time to blast open his cell door.
Battle droids immediately swarmed his position, but adrenaline and excitement at finally entering battle once more allowed him to disarm the first that arrived, using that rifle to take out the others. He took another rifle and fought his way to the torture chambers, where he exacted painful revenge on those who had done the same to him.
He then found the comlinks of the interrogators, which he used to locate the central communications area of the facility. He fought his way there, and send out a distress call to the Republic. He is received by a friendly force, which also destroys the facility itself, and is returned to Coruscant for debriefing and a reunion with his squadmates.
However, unknown to Sev, the Clone Wars are over, and he is now an Imperial Commando. His squadmates relate to him their new leadership and mission: to hunt down any other Jedi, in the name of Darth Vader. Sev feels some acceptance of this over the fact that it was Master Yoda's orders to leave him on Kashyyyk, yet he still does not believe it right to kill all Jedi for the acts of a few.
Whether Delta Squad remains to ruthlessly do the Empire's bidding, or becomes a mercenary unit against them, is a battle for the next game to decide (if only that were true).
My rating for the one above he is a 7.5/10 (maybe 8). I say this because I think it'd make more sense for the CIS to take him, not the Trandos, but that's just my opinion. Good idea though.