From Path of the Vampire Wiki
Prior to 2011, it was impossible to return from the Shadow Realm. Although the barrier between realms (Shadow and "Real") has always been thin surrounding Harper Rock, no vampire was ever able to break through that barrier and return to life. However, that all changed when Cobb, the last surviving vampire, "sundered" the barrier between the Shadow Realm and the "Real", effectively ripping a hole in the barrier (presumably, by mistake).The tear between the Shadow Realm and the "real' world is often called "The Rift".
The Rift is not a visible thing. It is an invisible tear between realities. The reality of the human world, located in Harper Rock and the reality of the Shadow Realm. Due to this tear between realities, energies that would normally be found only in the Shadow Realm, began to leak out.
This leak of supernatural energy had the effect of bringing dead humans back to life. The formally dead humans, now Zombies were now animated and moving under their own influence. This unexpected reaction caused the creation of the Quarantine Zone.
Another unexpected boon from the Rift was fadeportals. Fadeportals connect two distinct and unmoving points in space. When one passes through one end, they end up on the other instantaneously, as if they had been teleported. The power for the fadeportals come from the Rift. How far away from the rift the fadeportals will work, is currently unknown. The further away from the Rift, the less effective the power.