One of the players in my group is getting rather fond of a very powerful and clearly evil sword that gives them great increases to strength and magical ability. As a consequence, I would love to have them get possessed by the demon that inhabits the sword, but it wouldn't work very well unless this was kept secret from the other party members.
Is this remotely possible? Since everyone declares what their character is doing by speaking, I would have to sever this connection since the possessed player's actions would need to come from me instead, making it obvious that their actions were being controlled. I could take full blown control of everything, split everyone into different rooms, go into each one to hear what each players wants to do, then tell the other players what they actually end up doing, but this is way to much of a hassle for everyone and would make it rather obvious to the entire party that something fishy was going on.
Is there any relatively easy way to have a player lose control of their actions without the other players knowing it? Its fine that the possessed player knows they are possessed, but I want to hide it from the others. I fully expect the answer to be no due to increased difficulty it adds to inter-player communication, but I thought I'd ask anyway. I'm really looking for a system-agnostic solution, but if there are system-specific solutions to this issue I'd be interested in hearing them as well. :D