The Abyss is a vaguely defined and little understood set of dimensions that is completely unapproachable from the the other worlds. The world is only understood as the sources and home world of the beings that have a strong attachment with the Supernal and thus with any kind of magic.
Those beings are either the creators of the Supernal or its original manipulators. They have long abandoned this the worlds, yet the experiencing of Vulgar magic by Sleepers in the Real World, can cause them to perceive the Real World and the caster, causing the Paradox events.
Those beings are chaotic and always hostile.