I just read a little Fantasy Flight Games 's Grimm, and wanted to recommend it to you as an excellent source of inspiration for an Ars Magica Saga dealing with dark fae and stuff.
As far as I was able to ascertain, the book present classic fantasy tales with a grim variation.
This may be a good twist to put on players, who may recognise the "base" story, but will never guess what becomes of it.
It may also be a very good source of inspiration for travel to a dark aspect of Arcadia.
Well, Grimm fairy tales live up to the name.
"Then the wicked woman uttered a curse, and was so wretched, so utterly wretched, that she knew not what to do. At first she would not go to the wedding at all, but she had no peace, and must go to see the young Queen. And when she went in she knew Snow-white; and she stood still with rage and fear, and could not stir. But iron slippers had already been put upon the fire, and they were brought in with tongs, and set before her. Then she was forced to put on the red-hot shoes, and dance until she dropped down dead."
... the end of an 1892 translation of the brothers Grimm version of Snow White ...
surlalunefairytales.com/authors/grimms.html
Two hundred fairy tales to be read here. 
... but of course, they are mild compared to the oldest editions. Look up a book called "Grimm's Grimmest" for... evil ideas.
Look particularly for "The Children Who Played Butcher With Each Other".
I'm not sure it's on that list - or I missed it - but here's the essence of it: