The Tkon Empire was a massive empire that existed over 600,000 years ago. This empire was located in the Alpha Quadrant. The Empire had a population of trillions and was incredibly advanced technologically. The Tkon had the ability to move entire star systems, using planets as outposts for defense. Their empire collapsed not long after the Age of Makto, when the central star of their home system went supernova.
The Empire employed guardians known as portals to defend its outposts and to act as gatekeepers for those seeking entry to the Empire. At least one such guardian, Portal 63, survived and was encountered by the Federation starship USS Enterprise-D on a planet in the Delphi Ardu system in 2364. This occasion was the first instance of contact with the extinct Tkon.
Beyond some information regarding planets in the Empire and an understanding of their script and some of their history, little is known of the Tkon by the Federation. Even students of archeology such as Jean-Luc Picard had limited knowledge in the 24th century. (TNG: "The Last Outpost")
According to Terry J. Erdmann's Star Trek: Deep Space Nine Companion, the writers of the episode "To the Death" mistakenly referred to the Iconians as the Tkon causing the art department to design and produce sets based on graphics seen in "The Last Outpost". The error was discovered not long before shooting took place and corrected.
In the Star Trek: The Next Generation - Q Continuum novels, it is revealed that the Tkon Empire was destroyed after the being called 0, having "tested" the Tkon for the past few decades by inciting a civil war between the central Tkon homeworld and its outer colonies over the cost and effort necessary for the 'Great Endeavour'- a plan to replace their dying sun with a completely new star- triggered the collapse of their sun in a fit of pique after they survived his efforts and completed the Great Endeavour on schedule. This action resulted in 0's banishment from the galaxy by the Q Continuum, as well as the creation of the galactic barrier to prevent him ever returning. This also reveals that the Tkon were master solar engineers; at the moment when 0 triggered the destruction of their sun, they were about to teleport it completely out of their solar system, simultaneously replacing it with a new, fresh one, Q noting that the transporter array they were going to use to move the fresh star still exists in the present and recommending that Picard salvage it once he returns to his own time before someone else does.