The Nine Known Worlds are 9 distinct realms of space that were created when the Godstone was shattered into 9 shards. 9 unique realms were then created, each holding a shard of the Godstone and each shard still remembering from where they came. These realms can only be traversed with the help of the shards themselves when utilized by beings known as planeswalkers.
After great battles and wars between the planeswalkers, a peace was reached with the signing of the Shard Treaty and a council aiming to control interplanar travel. They built a shield called the cradle around the Nine Known Worlds and declared everything outside the cradle wildspace and forbidden.
Some places that offend the council are banished beyond the cradle. One such example are the Deadlands.