The Unity Device
The Unity Device was a piece of technology of Chodak design, which could be effectively classified as a space station, but one of incredible size, at about 314 million kilometers in diameter (approximately 2AU). It was greater in size than the Dyson Sphere the USS Enterprise-D encountered in 2369.
The construction of the device was mostly unknown, although it was spherical, almost hollow with a central control complex and internal areas with active life support capable of sustaining humanoid life.
At the time of its rediscovery, the Unity Device was situated in the Thang sector on the far side of the Z'Tarnis Nebula near the Romulan Neutral Zone, which would appear to place it on the edge of the ancient Chodak Empire.
Its separate components; I.F.D. (Integrated Field De-randomizer) and TAVAD (Temporary Artificial Variable Anomaly Device) have to converge and operate in unison in order for it to function as it was designed. Failure would result on a much disaster of larger scale than unbalancing matter and antimatter. (TNG video games: Future's Past, A Final Unity)
History
The device was built in the process of 1650 years. Its purpose was somewhat administrative; the Chodak government used it to control their far flung Empire, as well as occasionally using its awesome power to serve a military end. However a group of dissident Chodak grew increasingly concerned that by continually using the device they were doing irreparable harm to the very fabric of space and time, and urged an end to using the Unity Device for petty military purposes. They were however ignored, leading them to taking the drastic step of seizing the device itself in a coup remembered as the Great Treachery.
Based on rebel outposts, such as Frigis, they managed to reach the Device in the Dar Norgal sector. The rebel Chodak ship Grince jammed and disabled the Imperial Spaceliner Rinkyl and engaged the Device's chameleon field. Having fooled the Device's sensor for being Rinkyl, the Grince docked inside and the assault teams took hold of the Device.
During the reign of Emperor Corbulus the device disappeared; The rebels then sent it into the distant future, in the hope that when it came time to reclaim it, the Chodak would have matured enough to use its awesome power wisely. For this, the device would reappear in an interval of 456,000 balchacks.
In the meantime, the dissident scientists discovered that their worst fears were correct; the device had already caused severe damage to the fabric of the space-time continuum. Using the device for its original stellar engineering purposes would almost certainly result in the destruction of the galaxy, if not the entire universe, and a cataclysm could potentially occur even if the device was never used again. The scientists therefore allowed themselves to be absorbed by the device and formed a group consciousness, with each member responsible for healing the fabric of the universe in a different part of time.
With the loss of the device the Chodak were no longer able to control their Empire as they once had. The 'second class races' of the Empire rose up almost overnight, destroying the Empire and apparently being destroyed in turn, leaving little but ruins across the Alpha and Beta quadrants. However the Chodak themselves survived, fleeing towards the Galactic core and far side of what would become the Romulan Star Empire, finally reappearing in the 24th century as the time of the return of the Unity Device came and engaging in a vicious war with the Romulans.
Return
The Romulans in turn somehow found out about the Unity Device and raced the Chodak to the device, crossing the Neutral Zone in a large scale incursion -that cost them heavily- but successfully getting significant ships into the huge Nebula the Unity Device was hiding in, followed soon after by a Chodak fleet.
The USS Enterprise-D arrives at the Unity Device
Captain Pentara of the Asiram was also after the Device. She hoped to use it and restore order to her world after a revolution caused by the Garidian underclasses, inflamed by the Fifth Scroll.
Following evidence found on Frigis and Horst III, the Enterprise-D arrived at the coordinates of the Unity Device, and it found the enormous station orbited by the wreckage of the Chodak and Romulan fleets spread all around it, apparently having mutually annihilated each other. The Garidian Warbird decloaked to inform the Enterprise crew that in fact the Unity Device had attacked both fleets with some kind of defensive weapon and annihilated them as soon as they had opened fire with weapons in its vicinity. Shortly thereafter, another Chodak ship carrying Admiral Brodnack, the Chodak's champion to reclaim the Unity Device appeared.
The surface of the Unity Device
Since the Device did not fully exist in the time continuum, the temporal boundary surrounding it would disrupt transporter signals; all three captains (Picard, Brodnack and the Garidian, Pentara) descended on a shuttle down to the huge installation.
There the trio were faced with a series of tests to see if they were worthy to inherit the awesome power of the device. Eventually succeeding (in no small part to Picard helping the trio to work together and agree to use the device for the common good), the device, which had developed a kind of sentient group awareness, informed them that the Unity Device would no longer be used to settle petty political disputes, but to mend damage to the universe throughout both space and time, that could threaten the Galaxy. The device required one of the trio to join them to anchor 'this' point in space and time, Brodnak, accepting his peoples true legacy, accepted the challenge and joined with the device.
Shortly after, the device shifted out of this time period in a massive temporal explosion , beyond the reach of any race, with Captain Picard reflecting that he thinks Brodnak was the best choice to take up the duty of repairing damage to space and time that could destroy the Galaxy. (TNG video games: Future's Past, A Final Unity)
Power
The Unity Device was an enormously powerful device, able to somehow manipulate subspace in such a way that it could perform Galactic scale manipulations of time and space. The limits to its power were never made entirely clear, however Brodnack was emphatic that even a well meaning person making a slight mistake could cause catastrophic complications. After the Chodak Rebels took it into the future to avoid misuse, they melded with the structure and refused to use it for petty political disagreements, instead working to stabilizing time and space (assuming it wasn't used as such during the reign of the Chodak as well).
The device had a defensive system that responded with deadly force to any weapons fire in a certain range, even if not directed at it. High-energy plasma discharges devastated two fleets, not differentiating between Romulan or Chodak, who contested above it; a feat Admiral Brodnak scoffed at as 'the least of its power'. (TNG video games: Future's Past, A Final Unity)