I think we all may know exactly what this little trinket is as well.
For now, I have no reason otherwise to believe it is nothing either than that little knife that Matt "must protect" with his life, at "all" costs... I think we know why now. But why in the hell would the Travellers care whether or not this damned thing is destroyed, is it because it really will realease all the supernatural beings back into the world? Is that their only concern, it somehow seems improbable.
Did the Travellers originally make the otherside, as a group, as a place to put supernatural beings, because they found out to their horror a long time ago that when you try to kill supernatural beings something very bad happens on this plane, or in the "resting" place--heaven, or does something happen to the witch responsible for the creation of those supernatural entities? Or is something far worse going on in the background?
It'll be interesting to see why they need this protected so bad. Because it makes no sense why they would ultimately care if those supernatural beings came back (they will eventually, it is just a matter of time after all) OR why they would care one iota about a love affair gone wrong (except that "Tessa" seems to be abusing magic like a heroin junkie that just was given the keys to Fort Knox)? Somehow, it seems like there just may be a larger picture or story to the otherside; maybe it was a prison for something really, really, dangerous and they made it for that purpose...
Hopefully we'll get more information soon about the otherside and what exactly is all there.