Almost nothing is known of the ancient nation of Tarra, as it was destroyed in its entirety millennia before modern memory. Even the oldest residents of the Null Labyrinth, ancient undead and elves, remember little of the civilization, and at most they remember the end times of its fall. The only remnants of Tarra which remain to the modern day are eerie ruins within the Null Labyrinth, and scars of ancient magic from the civilization's height.
At its height, Tarra is thought to have extended through nearly the entire Null Labyrinth, possibly even into other subterranean areas such as Buyuk and Itlan-Youali, though evidence for these theories is scant. They were heavily isolationist, as surface nations never knew of their existence until recent contact with Null Labyrinth denizens and archaeological expeditions.
It is thought that Tarra as a nation existed about 4,000 years, from the early fifth millennium SF to the early 8,000s. They are thought to have been initially founded by populations of surface races who, for a variety of reasons, left the surface and moved into the vast network of subterranean caverns which spiderweb beneath Cahyali.
What little is known of Tarran culture shows that they were an arrogant people, believing themselves to be masters over the forces of nature. Their architecture is eerie and unnatural, built in jutting geometric shapes constructed from dark, hard materials such as stone and sometimes even obsidian. Pyramids are a common feature of Tarran buildings.
They also seem to have venerated the humanoid form, as many statues have been uncovered which are minimalist depictions of the basic humanoid shape. These statues often function as centerpieces of locations, usually holding an orb of some sort in their hands which is thought to have once been a magical lighting device, or perhaps a speaker of sorts to call a community gathering.
Many parts of Tarran architecture are inscribed with their script: a geometric, runic-looking script which is unfamiliar to most. Imortuii speakers have found slight similarities to their own script, though Tarran is missing many of the elements which make modern Imortuii understandable; the scripts simply bear a superficial resemblance. The only translation of the Tarran script which has been performed is thanks to the magic of Vormaxia.
These translated portions appear to mostly be poems, many of which commemorate the building they are built on, give some hint as to the function of a mechanism, or venerate Tarra as a whole. It seemed as if the ancient Tarrans thought their civilization would last forever, safe beneath the earth.
From the beginning, it appears as if the Tarrans were a portal-building culture. They were disinterested in colonization, instead exerting mastery over time and space and punching through dimensions with the intent of gathering resources and improving infrastructure across and between their vast cavern networks. The portal frames found in the Acheron Pits have lingering magical energy, with signatures varying from the Beneath to the Dreaming, and even Jaern. Several other hypothesized Cefa'zisto worlds are also thought to have touched the portals.
Many portal mechanisms' ruins are covered with a strange, rootless flower-like organism and are dripping with a caustic black ooze, which has failed to be successfully analyzed by any magical or physical techniques. Archaeologists studying Tarra mostly agree that these are extraplanar, even perhaps being voidic in origin. These flowers resemble those told about in gnomish legends; meaning they have some connection to the calamity in the early 8000s which forced the gnomes from the Dreaming back to the Cefa'zisto.
The strongest theory as to the fall of Tarra hypothesizes that, at one point, the portal-builders tunneled their way into the wrong plane, or in between planes. Some force or entity there retaliated and invaded Tarra, steadily destroying the entirety of the civilization and even pouring through any portals the people opened in attempts to escape.
One theory additionally states that the strange spatial state of the Null Labyrinth was triggered as a last-ditch Tarran failsafe - all intra-Tarra portals were collapsed at once, destabilizing the entire Labyrinth's place in physical space and leading to the ever-changing state of its locations. This is supported by the fact that the only fixed areas within the Labyrinth appear to be major Tarran ruin sites.