Transcendentem is a barrier encompassing an immeasurably infinite number of different Beyond Bubbles. The purpose of Transcendentem is to separate these Beyond Bubbles from each-other, as two Beyond Bubble merging would have apocalyptic consequences.
Reality is so fundamentally different outside Beyond compared to inside it that should two Beyond Bubbles merge, it would cause unfathomable amounts of chaos as the two contradictory forms of existence destabilized each-other.
There is an entity, stronger than everything in every beyond-iteration in Transcendentem, but not as powerful as some Aspect Lords, named The Guardian Of The 5th Wall who maintains Transcendentem and prevents any and all Beyond Bubbles from merging and causing such chaos.
The wall itself is equally different from reality within Beyond, such that any entity who exits their Beyond Bubble and enters Transcendentem without sufficient power will suffer the same fate as those who find themselves in two merged Beyond Bubbles. (Such as was the case for Venrensorys.)
Some entities have mental and/or physical power so great that they are able to comprehend and exist in the void between Beyond Bubbles, like Fantazia, however entities such as Fantazia can only travel through Transcendentem, they cannot properly affect or control it. A far greater level of power, such as that held by the Guardian of the 5th Wall, is required to do so.
Stage I Seeds from the External Hazard are very capable of moving through Transcendentem and a far greater number do than we realize. They fly through Transcendentem aimlessly, hopefully hitting a Beyond Bubble and latching onto it, however considering the absolute size of Transcendentem, it is likely the majority never hit a Beyond Bubble and just continue moving through Transcendentem with no direction.
Knowledge of Transcendentem
Knowledge of Transcendentem from within our Beyond Bubble is very limited. The Supergod Council know, mostly via Unthil's powers, that Transcendentem exists and that it contains infinitely many Beyond Bubbles like their own, however they do not know any of the details. Unthil's Omniscience is great enough that he could learn of them, but he chooses not to, as he knows the sheer fundamental difference between our Beyond Bubble and the rest of Transcendentem would destroy his mind. Thus, they choose to know nothing of Transcendentem other than the mere fact that it exists.
Other entities below the Supergod Council usually have no knowledge of Transcendentem whatsoever, even entities such as Creator and Destroyer truly believe our Beyond bubble to be all of existence. However, there are some exceptions.
The Omniversal Traveler has heard of rumors of the space beyond the Omniverse just being bubbles within a greater space, like a Universe to a Multiverse, and was motivated to keep exploring, however, the traveler lacks the abilities and the equipment to survive in just Beyond for a short time, let alone Transcendentem, a realm so fundamentally different from the form of reality The Omniversal Traveler is capable of existing in that merely exiting his Beyond Bubble would completely destroy him. However, this will likely not happen as The Supergod Council, even the sympathetic members, would not allow the traveler to go beyond their own reach, as even the council is unaware of what lurks beyond their Beyond Bubble. The fact that entering Transcendentem means that Hellensoerensen wouldn't be able to affect the traveler really says something about the full size of Transcendentem and the power of The Guardian of the 5th Wall.