I've been waiting a while for this one. Just a casual look at my anime collection and on seeing both Death note and Code Geass taking up large parts of it, I'm a fan of the just as planned type of move. A big one. This is no secret as elements have come out in several games of mine. What's a secret about it is normally the how.
Yesterday, in one of the final sessions for my current game (not including at least one more combat will (hopefully) take) I finally got to reveal the major one, revealing that an NPC the players have been around for a long time is an avatar of Nyarlathotep, and oh boy how fun this has been~
For the non Cthulhu mythos inclined, Nyarlathotep is the only vaguely human mythos god, able to interact with mortals in ways beyond murder and awe. Talk with them. Manipulate them. Hell, it happily violates that seeming rule of evil omnipotent things that it'll happily just lie. It's also one of my favorite mythos beasts as like the Colour out of Space (a creature that is colour and nothing more or less) its truly alien. Rather then a single form it's entirety is thousands of individual creatures, some completely alien, some just mortal, all aware of each other if not sharing a mind and working towards their goal.
The avatar in this case was the innocent little roommate of one of the players. It took her being very unsubtle before they finally confirmed that she's at least bad, the actual extent of it was only revealed in last week's where game one of my players DID make the connection but more as a joke then serious.
Initially, they weren't too sure either, she transferred into their should at the same time as the designated villains, after all, but she almost immediately won them over with her being so nice and helpful for them. She had no idea about their magical adventures fighting mythos things so who could blame her if she occasionally said things that, if she new the context for, would be mean, belittling their every achievement, pointing out all their failures, she even through a nice party to cheer everyone up after something very depressing. It's not her fault that it happened to be bad and depress everyone even worse.
She was also so very helpful. If it wasn't for her, the group would have been caught if she hadn't reduced another NPC to tears and provided them a distraction, and helped them get started looking for their next plot critical maguffin by helping that same NPC commit suicide, right in front of the most cowardly and distracted of players, scaring them into action. She's kept them motivated on their holy quest by helping the players friends be in the right place for them to be killed horribly or go insane from seeing friends die, or at least wind up slightly maimed by falling knives so they can't be distractions. The fate of the world is on the line, after all.
Of course, they were a little mean to her back, but she'd always get totally not disproportionate revenge by ditching them in a bad part of the city after getting them in trouble.
It took the accidental stabbing before they considered her a genuine threat, I think, but even then she nearly convinced them she was innocent and it took out of game prodding to get them to push this further. Even then she was considered trust-able literally right up to the line where revealed she wasn't.
Could not have asked for it to go better. I have no idea how I managed it, I'm not a good nor a cunning GM and my players know this is in line with my style pretty well, but somehow I did the right thing by tricking them. I wish I had a vague idea, as with tabletop games there are ways to do things that are very consistent for manipulating players without their knowing it, and I feel I might have stumbled on one by accident.
Other avatars of Nyarlathotep in the game rely heavily on claiming it all went as planned after it happened because unless you're brilliant (which I am not) that's the only physical way to make things look destined when you have something as chaotic as PCs in a system, but somehow this character who was meant to be pretty much an annoyance in place for a very distant singular role worked so well, every step of the way.
If I ever do figure out how I did it, I'll be sure to say here. I'd like to put out a list for those types of things here, it'd be a good topic to talk about. And handy for me to keep on mind.