Okay, here's the full low-down on Dunkelzahn, the Horrors, and pretty much the entire backbone of the first 10 years or so of Shadowrun's metaplot. Please note this will spoil a lot of adventures, sourcebooks, and novels that I highly recommend you read because they're really good. I'm not even going to block out the spoilers because that's all this entire post is.
In the 4th World (aka Earthdawn), the world was ravaged by beings called Horrors (they are never once called this in Shadowrun canon, only "The Enemy" by Harlequin and that's the closest anyone comes to naming them). The Horrors are basically Lovecraft's nightmares/wet dreams in some interpretations, but I personally think that the Skinwalker from the Dresden Files novel Small Favor by Jim Butcher sums them up best. They're hard to kill and they feed on human/metahuman fear and pain. And they are epicureans too. Sure, there's the instant screaming terror sort of fear from blowing up someone's house with their family inside, but that's like fast food. Tasty, but no depth of flavor and it's not that filling for very long. But that slow physical and emotional torture? That's the stuff right there... And there are billions of them. Billions upon billions, just waiting for mana levels to get high enough to come to the world and feast. The trick is (and this is important) that the mana levels have to be very high to get here, but they don't have to be very high to stay here.
Fast forward to 2018 or so when the Great Ghost Dance happens. Who gave them the info on how to do it? No one knows. But performing the ritual caused a "spike" in the mana levels. Imagine the gap that the horrors have to go through, and it's the Grand Canyon (not my images, it's how it's depicted multiple times). The Great Ghost Dance made a sort of stone bridge jutting out from our side to the side with the Horrors. It means they can come early, which is a bad thing (Understatement of the Year award candidate!). Blood magic also makes this bridge even longer, allowing the Horrors to come sooner. Key sign that things are bad, the Insect Spirits were about a century or three early for the mana cycle.
Fast forward to 2055 or so. Harlequin goes on a metaplanar quest (well, he doesn't really do crap except watch the backs of the poor runners who do all the hard work) against hordes of Horrors who were able to bridge the gap and sneak over. After the quest is over, the Horrors are stalled but not stopped (unless you ask Harlequin who thinks he single-handedly saved the world). Souce: Harlequin's Back adventure and the novels House of the Sun and Worlds Without Ends.
But there's a problem. Aztlan/Aztechnology is actually working to make the bridge longer through ritual sacrifices. Dunkelzahn tries to warn everyone who has the power and knowledge to do something (as depicted in the Aztlan sourcebook), but apparently they don't do a damn thing because Dunkelzahn decides the best course of action is to blow himself up. He does and uses his will and some documents left behind to send a run to Aztlan who is attempting to create a Locus (think Off-brand AAA battery : Power Focus :: Fusion nuclear reactor capable of powering the entire East Coast : Locus) by sacrificing several thousand people in a Blood Magic ritual.
Sidebar: See, Horrors can do nasty things to people to torture them. One of them is corrupting the individual to the point they're kinda a mini-Horror themselves. Rumor is that Aztechnology/Aztlan is run by a Horror-touched Great Dragon, which is a Bad Thing (yet another Understatement of the Year candidate).
One of the side effects of what Dunkelzahn does is create a Great Form Free Spirit that's as powerful as a Great Dragon. This spirit decides to possess the body of a cyberzombie...and gets trapped by the rituals to create a cyberzombie. Long story short ("Too late!"), the cyberzombie/spirit combo manages to drain the magic out of the bridge, shortening it so that the manacycle's back on target as normal. The spirit then becomes self-aware and realizes that it is in fact the soul of Dunkelzahn...but does this after everyone else has left. Oh, and Ryan Mercury finds out he's a drake. Seriously, it makes sense in context, that's why I said to read the novels (Stranger Souls, Clockwork Asylum, and Beyond the Pale).
Anyway, Crash 2.0 happens and Fanpro's management decides they want to kill that whole thread even deader, so they dropped a plane on Cyberzombie Dunkelzahn's Ghost. So don't expect it to ever come up again, especially since Horrors are an Earthdawn thing and that brand is now managed by a different competing company.
Also, Shedim have nothing to do with Horrors. That's a completely different can of worms that took the Old Guard (beings that had been around since the 4th World) by surprise.