Halo is an Acolyte Assassin played by Sion in Winson Paine's Candle in the Black campaign.
You see some incredible stuff out in the void. You see things that those in Hiveworlds wouldn't even believe even if you showed them a picture of it. You see beautiful, stunning things that leave your heart bleeding but you also see other things. Dark things. Ugly things. Things so foul, horrid and undeniably awful that even if your family are traders between worlds trying to eek out a life as civilians you have to eventually close your stall and take up arms. Such things are trouble that civilian traders do not go looking for but, out in the infinite expanse of the void, trouble sometimes comes looking for you.
The ship docked on Agrippina 3, a couple of planets over from The Forge World. A simple pick up supplies and move them to the forge job. The world was populated mostly be ferals but an imperial mining operation had been set up. The ore was loaded on to the ship and then it took off. A two day trip to the forge world. Two days was more than enough time for the stow away to make his way through the ship.
A feral unable to comprehend what was happening around him. Unable to see that the universe beyond his own small, putrid pile of shit he called a planet regarded him the same way most people regard single molecules of air (that is to say not at all). Unable to understand that those on the ship were not kidnapping him, stealing him or his soul. Unable to understand the pleas for mercy as he beat the women to death with the bloody end of her own rended arm.
The feral finally died to a shotgun blast to the back of the head. The rest of the crew were dead and the feral was facing out of the ships cockpit looking at the myriad of stars. The smoking hole in the beast-mans head left a red, white and gray mess on the window and instruments. All of sixteen years old at the time Triantor found the final blast easy. Growing up in the universe and seeing all these things put everything in perspective. In the service of the emperor you were but nothing. The feral had killed traders for the empire. It had killed his family. He had killed the feral.
Triantor calmly piloted the ship into the docking bay at the forge world. A day on board a ship with a dozen corpses had left the boy looking whiter than he did normally. His skin color never quite returned leaving him to a deathly pallor. He walked out of the ship calmly and moved over to the dockmaster.
"I'm afraid there was a feral was a stowaway on the ship. He killed the crew and almost stole the supplies but I brought them here," he said. Everyone handles grief in different ways. This was the void-dwellers way and he didn't expect a Hive Worlder to understand. How could he? He hadn't seen anything like it and he wouldn't believe it even if he was shown a picture. "It's okay though. I killed the feral scum."
It was about this time that the dockmaster summoned a member of the inquisition. A hive worlder. Thin. Old as some of the buildings on the planet. He took Triantor in and turned him into his own pet project. Six years of training and he's been released from his carer, a veritable pit bull for the emperor. He never openly showed his grief over the loss of his family or remorse for what he did. Inert, neutral, calm and detached. Perfect for the Officio Assassinorum.