Reality Beta is the origin place of the creatures (such as the Behemoth and Shikari) attempting to penetrate normal reality (designated Reality Alpha) in the game Blood II: The Chosen. The name is non-canonical and is never used in Blood II, and is instead used by the Blood Wiki to differentiate the different realms. Only Ishmael ever refers to it, and simply calls it ambigously as "another reality". The player enters this reality in chapter four of Blood II: The Chosen, in the level C4L1: Beyond the Rift.
There is a brief mention of an otherworld in the Death Shroud's offical description and during Gideon's narrative in The Nightmare Levels. However, there is also a place called the other world in the Cryptic Passage expansion for Blood, where Caleb fights the Lord of All Nightmares in E5M9: Castle. Although Cryptic Passage was released before Blood II: The Chosen, it was also not designed by Monolith Productions, which makes its canonicity somewhat dubious. The evidence that this other world is the same as Reality Beta from Blood II cannot be corroborated (and would seem to contradict the storyline of Blood II).
The Blood II website, wary to provide spoilers, refers to this reality and its inhabitants as the "unknown", making the remark "what you don't know can hurt you."
The descriptions for the Phantasm from Blood make mention of a "land of the dead", a possible third reality which would presumably have housed the souls of the Chosen prior to their resurrection via the Singularity Generator within Blood II (and possibly Caleb before his ressurection in E1M1: Cradle to Grave, as the Phantasms intently stalk him afterwards).
Reality Beta's landscapes are largely made up of rocky outcroppings and deserts. There is also an ancient citadel inhabited by Drudge creatures infected by Bone Leeches, though a conspicuous Cabal symbol is seen on a gong in the main temple (possibly placed there by the CabalCo expedition teams in anticipation of Gideon's arrival). This may indicate some sort of collective social structure in the Drudge beings, as they have one in their biology, like bees. The official descriptions for these creatures claim that Drudge creatures have a social hierarchy based on maturity (Soul Drudge, Drudge Lord, and Drudge Priest). The Drudge Priest's can wield dark magic, just as the Cabal can in our own reality, mirroring the Zealots. The Bone Leeches are perfectly adapted to taking over human bodies, which may indicate that they evolved as a parasite for a similar being in Reality Beta, which may have been responsible for building the citadel even though they are never actually encountered.
The Ancient One appears to be the "ruler" of this world - at least it directed the actions of other creatures in invading Reality Alpha. This began due to Caleb neglecting to use the powers of Tchernobog to bind the realities and keep them separated. Cabalco's use of the Singularity Generator also had an influence on the portals being allowed to open, but to what degree is unknown. In the game, portals do not start opening up until after the first time the Singularity Generator is used, but the Cabalco Enterprises buildings show several established experiments with the other worldly creatures. It is likely that this device is responsible for breaking down the bindings of reality - since there were no known rifts in the a century between Blood and Blood II - and only Caleb had the power to repair them.
Invading creatures consisted of Bone Leeches (which had the power to infect and transform humans), Shikari, Behemoths, Death Shrouds, and the Naga. It is unknown how long this breach of dimensions lasted, but CabalCo was able to use the creatures in experiments in its laboratories, and sent scout teams into Reality Beta - though the large amounts of bodily remains scattered around (and one team's shown encounter with a Behemoth) make it quite clear that they never reached very far out into the surrounding citadel and fewer still returned to report.
Gideon met his end in Reality Beta, after being followed there by Caleb, who later destroyed the Ancient One, ending its invasion. Afterwards, he presumably seals the rifts and returns things to normal, although he and the other Chosen remain in Reality Beta. During their "long walk home", Caleb was abducted by the Nightmare, and forced into different "visions" of the past. Caleb defeated the Nightmare, but it is unknown if Caleb ever makes his way back to the Chosen, or if they make it back to our reality.