The Veil is the border that separates this world from whatever lies on the other side, acting as an impassable barrier. It is the true, tangible end of the world - though it cannot be seen or touched normally.
Unfortunately, the Veil is not impenetrable. Damage can occur in the form of Rifts. Powerful beings and events have been known to split the Veil, tearing open holes through which inevitably demons enter. Unlike the Veil, Rifts are visible - they appear as huge, swirling vortexes of darkness that levitate above the surface. The ground of the surrounding area is barren and dead, scorched like the result of a localized fiery cataclysm. The air smells of smoke and though dark clouds never part above, it never rains either. The site emanates an unpleasant, foreboding feeling of wrongness. Fortunately, Rifts can be mended - though this act requires an immense amount of power and focus. Closing a Rift also restores the damage done to the environment that took place on its opening, albeit over a long time.
There are two main authorities that seek out and mend Rifts - the mystical and secret Hellguard, and an order of mages who call themselves Rifthunters. According to the statistics reported by the Rifthunters, an average of 0.3 Rifts opens up per year. It is currently estimated that there are at least three active Rifts on the continent of Asgard.
Nothing has ever come out of a Rift except demons, and though many have gone into Rifts in hopes of discovering what lies beyond the Veil, no one has ever returned to report their findings.