Shadow magick to some has different meanings though the main ones people tend to think of are in essence shadow based kinesis which is not possible. I've heard some say they are capable of darkening a already dimly lit room but this can be attributed to a strong imagination considering the room in this situation is already dimly lit. I can't fully recall the conversation yet I have heard shadow magick somewhat related to manipulating ones emotions or harnessing darker emotions, but shadow magick isn't a topic I've seen discussed often.
I have spent considerable time on shadow or dark magic and some call it unnatural and dangerous but its wat i do id recommend that new people start learning magic in the path of their kith or innate talent and some of shadow magic can be very dangerous as it leads to very dark places and is not studied light heartedly
I don't do much on shadow magick, at least not the kind your talking about and I'm not even sure what I do can be classified under shadow magick, but I know there are books on it. Look at 'New World' books and see what they have. Or just type it up in a search engine and I'm sure you'll find a good book.
I know of the side effects Ragnorrok, I have somewhat of a knack for it, but didn't really have much experience in control. I would love to hear about your research Ragnorrok, if I may. NimirRaj on the fact of dimming a dull lit room, I think it simply takes a lot of concentration to do so, anything is possible with enough will to enforce it. Thanks ThoraStorm I'll look into it.
I appreciate everyone's help and opinions, may your gods bless you.
On a broader perspective, I would suggest you read up on Carl Jung's theories of the Shadow Self, to learn about how that pertains to you, and how you can use to to empower yourself in your magickal and self developmental path. :)