I Like the Suprise Ending
CONTAINS SPOILERS
A lot of people are very unsatisfied with the ending, a.k.a. finding out the movie was really about an "alien", not a ghost. but i think it works really well, and kind of separates this movie from all the other horror films that simply have a ghost haunting a certain location because they died tragically at that spot you know blah blah. and you would never really suspect that karen will be brought back; i was just assuming they would find out how she really died and that would be the big climax.
As for the watcher's true identity, I think this watcher was just as lost and scared on this world as karen is in hers. Really everything the watcher did was to help others: saving Jan from being hit by that motorcycle, saving Ellie from being hit by a truck, allowing Karen to be freed. The watcher was just forced to wait in the forest until another eclips. I think the whole seance, and what exactly they say while holding hands, is not necessary to do. The watcher just needs an eclipse for the "magnetic pull" as they put it, an image similar to Karen's in order to bring her out of suspended animation, and the power of several people holding hands.
Loved the movie!! If you are a fan of the scary movie genre, this is an excellent one with a superb plot.