The Portal in the Forest
The Desolate Guardians
The Moon Aflame
'I didn't miss the end of the world [Final Part]'
Our Final Acts
Described visually as a two dimensional tear in space that leads to a parallel universe. Originally an oval blur, the original or primary portal in the forest began to grow and become unstable due to traversal by the multi-dimensional Soul Reader (Book) device, coupled with the instability in the emotional state of Thomas, the Brownshirt child generating the original portal. This instability can be limited by the portal being closed to allow space to heal.
These portals appear to be limited in their ability to transfer matter and energy between worlds, and will typically only allow matter or energy to originate from one side. The visiting matter can return through the same portal, however matter and energy from the destination side does not appear to be able to traverse the portal without being deliberately retrieved or taken by something from the originating side of the portal. This "memory" effect exists only with a given iteration of a portal, with any traveler who is left in another universe when the portal is closed stuck there unless a new portal is opened from their side. The limitations of bidirectional energy transit do not appear to apply to light, as observations can be made through the portals from both sides, albeit with slight blurring.
Currently, the only known creators of such inter-dimensional portals are the descended energy beings known as the Brownshirts. Other entities also appear to be able to traverse parallel universes, such as Sentient Flames. The Regret Demon is capable of hijacking an active portal and redirecting its destination to a location of its choice. Techno-Portals can also be generated by the Portal hub facility installations, which are capable of targeted inter-dimensional travel.