Fieras is probably the single most unpleasant and inhospitable system to ever be classified as "habitable by humans". Since 2196, it has been the site of a continuous and ongoing three-way battle between the Bugs, the Tech Infantry, and an especially nasty local life from known only as Soul Eaters.
Its lone life-bearing world is Fieras VI, also known as simply "Fury". It is a world whose intense gravitational and electomagnetic anomalies cause most forms of electronics to malfunction, especially anything like navigation or guidance systems. Extreme weather systems and the rough terrain produced by an especially active tectonic system compound the unpleasantness of the place.
The Soul Eaters
Although most of the local flora and fauna can and will actively try to kill you, and all of it is poisonous to eat or drink or even brush against, by far the most dangerous are the Soul Eaters. Intelligent, devious, and intensely magical, these nasty parasites can not only reshape your flesh into nightmarish forms, but can twist your mind into madness and, in the end, totally replace your personality and memories with its own and completely possess its victim.
The Long Campaign
The Tech Infantry first landed on the planet in 2196, expecting just another campaign to clear a world infested by Bugs. What they found was hell. The intense magnetic and gravitational anomalies played havoc with their electronic systems and left them without many of the technological wonder-weapons they relied upon to even the odds in their fights with the Bugs. The presence of Soul Eaters was simply the icing on the cake. But as long as there remained a Bug infestation on the world, it was a threat to all the nearby star systems, so it had to be taken.
In the late 2240's the government of Arthur Clarke seriously contemplated a scheme involving constructing in orbit over the planet a gigantic mylar disc to block sunlight and put the planet in permanent shadow until its atmosphere literally froze solid. But even if that managed to kill off the Soul Eaters, a far from certain prospect, it was unlikely to be more than a minor inconvenience to the Bugs already entrenched in their underground tunnel networks. Attempts were made to make the star go supernova, but the electromagnetic anomalies in the system rendered the Dooms Day Device inoperative and that plan too had to be abandoned.
Behind the Scenes
Fieras is named both for the homeworld of the Mrrshan in Master of Orion II, and for the planet Fiorina "Fury" 161 in the movie Alien^3. In keeping with the latter, a prominent feature named Sigourney Ridge is named for Sigourney Weaver. The long, hopeless campaign there is inspired by Vietnam, World War One trench warfare, and every other campaign dating back to the Trojan War where each side spent more time fighting the natural environment and the incompetence of their own side than they did fighting their nominal enemies.