The End of Days by dansemacabre

Labyrinth

"Nineteen-year old Sarah is back home from college over Christmas vacation, and she's long given up on the Labyrinth and her friends as being just another fantasy she's outgrown. But when Hoggle appears in her mirror with a plea for help, she's dragged back to the Underground. Dire things are happening in the Labyrinth, and Sarah might be the only one who can save it... and the Goblin King."

I've read a fair amount of Labyrinth fanfiction and this is without question the best I have ever found. The characterizations are spot on and the plot is new and original. Highly recommended.

The End of Days