Pugar “The Magician” Arreon claims his big brother is a Warp Talon and that’s where he got the awesome helmet and chainsword; while the claim is patently ridiculous the sword is basically brand new … there’s no Nurgle rot, Tzeentchian sigils, and not nearly enough blood to have been looted from Khornish marines. Exactly how Pugar acquired is unclear.
In the revelry after a particularly successful battle another cultists, Cruga tried to insult Pugar by calling him a lumberjack, and Pugar responded by sawing him in half and then proclaiming that he couldn’t put Cruga back together so he was apparently a pretty piss poor magician. This struck a cord with the rest of the cult, and now there are usually raucous calls to “Show us a magic trick” from everyone else in his squad when they charge into battle.
Pugar has the distinction of having one of the few Chaos amulets not made by Moebius, though he’s not clear on the origins since it was attached when “his brother gave it to him.”
The sword and helmet are indeed spares from the Warp Talon/Lightning Claws kit. The pistol is actually from a Kharadron Overlord kit (aka fantasy sky dwarves).