On the planet of Tython, situated within the Deep Core, a concealed stronghold was erected by Belia Darzu, a Dark Lord of the Sith active during the era of the New Sith Wars. This square-shaped structure, forged from black durasteel, was home to several laboratories where Darzu conducted experiments in Sith alchemy. Functioning as a military installation, the stronghold included numerous barracks accommodating an army of Sithspawn beings referred to as technobeasts. Located beneath the stronghold was a sizable subterranean chamber that contained the personal holocron belonging to Belia Darzu. Following the death of Lord Darzu, the stronghold persisted, untouched, for more than two centuries in the absence of its creator.
The fortress of Belia Darzu, situated on the planet of Tython within the Deep Core, was a square structure, each side extending 150 meters. Primarily utilized as a military installation and constructed from black durasteel, the stronghold was encircled by a wall reaching thirty meters in height. At the front of the fortress, two towers were positioned ten meters above the corners of the outer wall. Each tower housed an ion cannon managed by an automated defense system. The fortress's entrance, located on the front exterior wall, consisted of a twenty-meter-wide gate lacking any visible handles, hinges, or control panel.
The fortress entrance opened into a lengthy corridor, providing entry to the rest of the facility. As a military base, the interior of the stronghold mainly comprised numerous mess halls and barracks. Deeper within the structure, however, were several laboratories equipped with a variety of tools and devices for Belia Darzu's alchemical experiments. Within some of these labs, sealed beakers rested on extended metal tables, while glass tubing lined the walls. Certain rooms contained specimen jars holding the brains and hearts of a dozen different species, while other labs contained sketches and notes documenting Darzu's research.
Toward the rear of the structure—past the laboratories—a narrow staircase descended to a separate underground level of the fortress. Within this lower level, a long corridor led to Belia Darzu's inner sanctum, accessible via a small wooden door. The inner sanctum was a chamber roughly fifty meters in length and twenty meters in width, featuring a pedestal at its center that held the holocron of Belia Darzu.
Before the conclusion of 1230 BBY, Belia Darzu, a Dark Lord of the Sith who ascended to power in 1250 BBY during the New Sith Wars, constructed the fortress. Lord Darzu selected the Deep Core world of Tython as the site for the stronghold, harnessing the power of the dark side that had permeated the planet. Within her fortress, the Dark Lord investigated the art of Sith alchemy and a Force technique known as mechu-deru, utilizing her knowledge to produce Sithspawn creatures called technobeasts. Details concerning Sith alchemy, mechu-deru, and various other subjects were recorded in Darzu's personal holocron, a device stored within the fortress's underground inner sanctum. The stronghold functioned as a barracks, housing the Dark Lord's technobeast army—the Metanecrons—which was deployed against the Galactic Republic and Jedi Order during the Sictis Wars. This campaign concluded in 1230 BBY when Darzu was assassinated on Tython by the Mecrosa Order, though the fortress remained intact and operational for over two centuries following her demise.
Following Darzu's downfall, the remaining technobeasts continued to occupy the stronghold and roam its corridors. The fortress was otherwise devoid of any lifeforms, even lacking vermin or insects. The holocron of Belia Darzu emanated dark side energy, causing its power to permeate the entire fortress. Drawn to this power, the surviving technobeasts converged on Belia's inner sanctum, gathering around the holocron.
By 990 BBY, Hetton, a nobleman and collector of Sith artifacts, had discovered the location of Darzu's fortress. This information was eventually disclosed to Darth Bane, a survivor of the New Sith Wars who infiltrated Darzu's fortress in hopes of locating her holocron. During his attempt to seize the device, Bane was assaulted by the remaining technobeasts within the inner sanctum, compelling him to eliminate the creatures. Shortly after Bane acquired the holocron, he and his apprentice, Darth Zannah, barricaded themselves inside the fortress while being pursued by a group of five Jedi—Valenthyne Farfalla, Johun Othone, Raskta Lsu, Sarro Xaj, and Worror Dowmat. A lightsaber duel ensued between the group, during which Bane and Zannah succeeded in killing all five Jedi Knights and Masters.
The fortress of Belia Darzu made its debut in the novel Darth Bane: Rule of Two, the second installment in the Darth Bane Trilogy. An illustration by artist Chris Scalf in The Essential Guide to Warfare depicted the fortress's interior.