Tallarn

Tallarn was once a verdant 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 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 hypothesised Chaos gate buried deep in the planet,or perhaps it was just a demonstration of the randomness of Chaos.