Two sessions ago the Rogue Trader in the game I'm running had cranial armour implanted. I asked the player what he had done with the skull now that it was no longer in the characters head, he replied that the Rogue Trader had it put back together and put on display.
Next session the Rogue Trader found out that the skull had been stolen and, while they think they know who stole it, that character is out of their reach and they don't know why he stole it. As the gm I know that it was stolen because a third party was offering to pay well for that specific skull.
But now that the Rogue Traders skull is in the possession of his enemies, I'm not sure what they can do with it. The only thing that I can think of is using it for a combat servo-skull and sending it to attack the players with sufficient taunting to let them know who the skull came from. But, if possible, I'd like to do something more memorable with it.