Tallarn

Tallarn was once a vibrant Paradise world, until it was attacked and Virus Bombed during the Horus Heresy by the Iron Warriors. Every living thing on the surface of the planet, even bacteria, was killed off, with only those who managed to seek shelter in special bio-isolation enviro-shelters deep undergound surviving. Shortly thereafter, the planet became the site of the largest tank battle in the Horus Heresy, with millions of war machines fighting over the planet's ravaged surface. The survivors learned to adapt to their world as it eventually became a desert planet. For what reason the Iron Warriors fought for the planet so hard is unknown, but it may have been to do with the Chaos gate buried deep in the planet,or perhaps it was just a demonstration of the namesake randomness of Chaos.

The planet was now devoid of all plant and animal life and most of the water had evaporated into the atmosphere. A thousand years later, a mysterious gate was discovered, triggering a series of sudden attacks by the enigmatic Eldar, who tried to prevent the gate from being opened. They were unsuccessful, and a tide of Daemons streamed forth from the gate. The Tallarns and the Eldar were quickly forced to work together in order to halt the tide and seal the gate once more. Gradually the combined forces pushed back the Chaos horde, the Tallarn people knowing the desert, leading the Eldar to strike at the most vulnerable Chaos points. Eventually they forced the Chaos forces back into the gate and managed to shut it down. The Eldar left machinery to keep the gate shut, and the Tallarn covered it with plasteel and turned their back on it. The gate is not closed for good, but will remain sleeping until it is called again.