Create a stable portal to another world or plane with enough energy to transport tens of thousands of powerful creatures, some of whom were at Rank 4, was an incredibly difficult thing to do.
Most worlds and planes have great resistance to any type of portals made that wished to create a connection inside them to another plane with exterior forces. And even if the portals were created, the amount of energy that any individual will need to use it will be immense.
But the Abyss was different. This was an evil and chaotic plane with an insatiable hunger to corrupt other planes, increase its domain and let the demons extend their claws into every world and plane in the universe.
