Splinter B is the broken split of existence that occurred during the May 15th incident in which Voidweb was released.
When .entropy was activated, its initial goal was to act as a ".reality" for certain portions of existence. This involved the Aethierium, Infinitium and Humanity along with other locations. Normally, beings are unable to grasp the perceivable universe, however, the Hethian blood powering it was that of both Zag'orv and De'ebo. This caused .entropy to overload and erupt within the fabric of the universe and caused it to splinter into two portions. Existence as a whole was split into Splinter A and Splinter B, with Voidweb beginning it's spread throughout Splinter B, away from the Arkn and Dekn in A.
Splinter B is the section of existence created from .entropy. The only living beings native to it are humans, within their own copy of a .reality core. Other than that, Arkn and Dekn and creatures were not copied over. Instead, Voidweb has unleashed beings and creations that take their place, along with the creatures it has already consumed and understood. The laws of reality are almost completely destroyed, which nullifies most sethe and completely eliminates Lurkers from the nexian cores. Because of this, humanity is not bound to .reality and can escape via methods introduced by the remaining R.O.T Hybrids that survived entering Splinter B through the Red Nova. This method allows humans to exist .reality and escape into the surrounding universe.