Alright, I was reading up a bit more on my 40k fluff and I have decided that your death cult assassin is banned for not making any sense, at all, fluff wise. Death-cults are part of the inquisition and only used sparingly. They are exceptional fighters hyped up on drugs, not particularly bloodthirsty, just drugged out on slaught and other horrible things. The inquisition would not send a death-cult assassin to a world the Imperium has already lost and forgotten.
If they wanted to prevent the fall of the planet the Inquisitor would just requisition an entire Imperial army and send it at the planet. If they wanted to kill a damned ork warboss or chaos cultist there are certainly better canidates for the job and all assassins used by the inquisition, including just normal death cult, are far to powerful to let a player of the RP control. So in conclusion, no more Sandara.
"Death cult assassins are often utilised by the Inquisition, the assassins' skill..."
They are apart of the Imperial Cult yes, but are often used by these Inquisitors. I didn't write that post a friend of mine did.
Anyways, I just don't think a Death Cult assassin would be stationed on a planet that is essentially doomed to die.