The Longest Journey
Also known as the "Rift", the Divide is the 'dream world' that separates the twin worlds of Stark and Arcadia. From what we've seen of it, the Divide just looks like an area of blue light. While dreams can and do cross freely between worlds, only a Shifter can physically pass through through the Divide from one world to the next.
The term 'Divide' is also used to refer to the event 12,200 years ago of separating Earth into Stark and Arcadia; this created the 'artifice of the Divide' as Gordon Halloway terms it. According to Vestrum Tobias, the Stone Disk of the Draic Kin was the 'epicenter' of the Divide.
The Divide was created at the instigation of the Kin, who had seen the worlds consumed by Chaos when magic and science were wielded together by unscrupulous humans. One of the Kin gathered 6 magicians and scientists (who were the first Sentinel) and a woman who would be the first Guardian. Then, using a disc they had brought from their world and the future Guardian's Tower, they enacted a ritual that divided the world into two. The Tower was now in the new Guardian's Realm, with an entrance in Stark's outer space. The precise 'location' of the Guardian's Realm in relation to the Divide and the Two Worlds is uncertain.
The only non-Shifter known to have traversed the Divide was Brian Westhouse, but he was trapped between worlds for 300 years before he finally appeared in Arcadia. He claimed that if he attempted to use the Shift April opened to return to Stark, he would "suffer a most unpleasant experience, and be lost in the between forever".
During TLJ the Vanguard and forces of Chaos were attempting to bring down the Divide and forcibly reunite the two worlds. This caused 'leakage' from Arcadia into Stark until a new Guardian was installed. It is suggested that Reunification of the Two Worlds was always planned by the Draic Kin, however, and implied that April will play some role in achieving this.
Brian Westhouse's journey across the Divide is now even more mysterious because the monks at the monastery were somehow able to send him - presumably across the Divide - to the newly-revealed Storytime, not straight to Arcadia as was previously believed. The Storytime appears to be the "world of dreams" between Worlds referred to in TLJ. It is not known how Brian reached Marcuria from there. Brian also described his hiatus between worlds to Zoe as being "unstuck in time", rather than being trapped in the Divide itself.
We further discover that, using a Dreamer Console, Zoe Castillo's 'dream presence' can pass through the Divide via the Winter with Faith's help. The Collapse may also have had an effect on the Divide: it is now possible for April to Shift directly to the Guardian's Realm.
It is not known if the Divide affects only Earth or the entire universe. Given April's visit to the Morning Star in orbit above Earth, it might have affected the whole universe. This would mean, however, that the home world of the Draic Kin would have been similarly affected, which may imply that they had a more personal interest in the creation of the Divide.
The vague reference to other "worlds" being destroyed by Chaos is equally mystifying. Whether this indicates other planets or entire past universes is uncertain.