This is the plane of negative energy. It is a realm devoid of life, light, fire, heat and electricity. No living creature can survive here for more than a few minutes as their life energy will be quickly siphoned away by the near-vacuum and infinite bleakness. The terrain here appears as a featureless black sky and a dim gray landscape of rolling dunes of sand or gritty snow stretching endlessly in all directions. There is minimal atmosphere, so noise and speech is muted. There is no light source, so no shadows; but a dim gray colorless light suffuses everything. As a result, this is a stealthy recursion, where surprise and ambush is more likely. Antithesia is also the realm of the living dead.
PC’s cannot exist here in human (or any other living) form, and as such will assume the form of an undead human (or humanoid body) in this recursion, as best fits their type and descriptor. PC’s here require no food, air, nor sleep; although restoring lost stat points during a short or long rest does consume time spent while inactive, and assumes the negative energy here re-infuses the undead bodies of the PC’s.
Because Antithesia is almost entirely devoid of sentient beings and apparently without boundaries, it is a place where rare super-powerful beings have historically chosen to locate a stronghold or hideout. Any landmark or structure in the endless landscape was probably created in the long-distant past, and can only be found by sheer luck, requiring a near-infinite time span spent wandering the gray desert, or by translation here with foreknowledge of the location. Random encounters with other NPC’s in the form of undead creatures, or with other wandering creatures (who could exist in this hostile environment) would be very unlikely but not impossible. There are myths and legends about portals to Antithesia from Earth located in ancient temples or abyssal dark caverns.
There are seven nexuses of dark matter on Antithesia and these are the most likely places to have been chosen as locations for structures, strongholds or temples by ancient beings. At each one, concentrated dark matter is present in a state that is physically manifested as a stream, pool, fountain, waterfall, geyser, etc. of black plasma, and this natural feature will have been incorporated into the built structure. The power of the dark matter streams is unknown upon living creatures, but upon the undead acts as a flow of water would upon the living.