So because a number of people have been interested in it in the past, this is a post where you can request that the Corinthian give your character a nightmare. He doesn't have to have even met them yet-- the Corinthian will pretty much ninja himself near or into the bedroom of anyone he senses in need of a "lesson".
Details about requesting and a nightmare FAQ are under the cut.
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NIGHTMARE FAQ: THE HELL ARE YOU TALKING ABOUT?
I. What's The Point?
As fun as inducing mental trauma can be, the Corinthian doesn't just do this for kicks-- he has a specialized purpose. The Corinthian's primary function is to strip away delusion. In his master's words, he was "created to be the darkness, and the fear of darkness in every human heart. A black mirror, made to reflect everything about itself that humanity will not confront". Those things that people deny about themselves, the things they've done or faced that they try to repress or don't want to admit to, he reflects. He forces dreamers to see what they hide from themselves in their waking hours. This can mean anything from making a serial killer who thinks he's murdering for a grand purpose realize that he's on the same low level as a rabid dog, to forcing Mary Sue to realize that the hate she harbors for others only masks her own insecurities. So long as the Corinthian is stripping away pretty lies to reveal the ugly truth, he's performing his function.
II. How Does He Do It?
Since canon never actually shows him doing it, I've had to speculate. It's assumed that he was able to reach dreamers on a mass scale when in the Dreaming, but since he's in Siren's Port I've decided that he's limited to affecting only a handful of people around him at a time, and they must be in relatively close proximity to them (i.e. walking through the hall of an apartment building, he could reach into about six or seven rooms at once). That is, if you'd like to have your character nightmared, he has to have a way to get near their bedroom or wherever they're sleeping.
As these nightmares have a very personal focus, and canon doesn't show the Corinthian being able to see into every detail about a person's life, my conclusion was that he directs nightmares, rather than writing detailed scripts. The Corinthian draws the basic premise of the nightmare from the person's own subconscious, which is just my roundabout way of explaining that no, he's not some creepy little god creature who knows everything about your character. However, he is privy to the events of the nightmare, so keep in mind that if that past events are replayed or secrets displayed through the course of the dream, he'll have knowledge of them afterward.
III. Aftereffects?
There are no physical consequences to being nightmared. It's just like having a bad dream: it's over when you wake up, and besides the normal bodily reactions to a nightmare (fast-paced breathing, rapid heartbeat, momentary disorientation or-- if it was traumatic enough-- crying), nothing happens. Your character will not start bleeding from various orifices or anything.
IV. Will My Character Know It Was The Corinthian?
It depends. The Corinthian's nightmares are no different from regular ones, if a bit more visceral. So if you choose to have your character remain asleep throughout the experience they'll remember the nightmare when they wake up, but won't be aware that it was because the creepy white-haired dude was near their room. Characters are of course free to suspect the Corinthian if they've met him before and are aware of what he is, but this depends on how likely your character is to jump to conclusions.
On the other hand, you can also choose to have your character wake up from their traumatic nightmare (as people sometimes do), in which case, yes, aforementioned creepy white-haired dude would still be somewhere near their room (possibly even in it). I don't mind if this happens, but be aware that the Corinthian will be unapologetic for his actions, and if attacked may fight back with the intent to kill. He's unstable like that.
As far as characters with dream powers/dream sensitivity/psychic powers are concerned, they may have a vague feeling in the nightmare that something is Not Quite Right. Please exercise this awareness intelligently-- The Corinthian is a literal nightmare, meaning that he's pretty darn skilled at manipulating dreamers and avoiding notice. Chances are good that these characters would only get a vague sense of his presence, and chances are even better that they wouldn't sense anything at all.
V. Why Would I Want To?
Masochistic tendencies? Actually, having your character forced to confront something about themselves can be useful. It's up to you whether the Corinthian's lessons have any permanence (some characters are understandably more stubborn than others), but if your character has an Issue of the denial nature that you wish they could build a bridge and get over, a nightmare might help.
Plus, you know, it could be fun. :D
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That's pretty much it! If you like the idea just comment below, PM (private message) me, or catch me on AIM (homocycle), and we can discuss details. These posts generally take some planning, as I have to know what the Corinthian will be "teaching" before helping to come up with fun dream symbolism and scenarios.
If you still have questions about the nightmare process or the Corinthian, please feel free to comment and ask so I can answer and add it to the FAQ.