Therone Hive War

The Therone Hive War was a xenocidal Imperial campaign carried out by the 501st Cordanian MIR alongside other Imperial military forces against the dire threat of a voracious Tyranid splinter Hive Fleet upon the vitally important hive world of Botrus IV. Though the Imperial forces put up a valiant defence of the doomed world, in the end they were forced to enact Exterminatus in order to deny the Tyranids much needed biomass to continue to function.

History
A small splinter hive fleet had come upon the hive world of Botrus IV. Fortunately, one of the Cordanian battle fleets was in the system when the hive fleet invaded. However, they only had a few troop carriers that have a few platoons of the 501st and the 111th at the time. These two units dispatched their scout ships out to answer the call for reinforcements. Even if it was a small splinter hive fleet, they were comprised of potentially thousands, or even millions, of Tyranid bio-organisms. These Astra Militarum regiments had to hold the main infection areas where the Tyranids made planetfall until they could receive more aid. It took several days for the scout ship to answer the beleaguered world's call for aid to be sent outside the range of the Tyranids' Shadow of the Warp null-effect. But now the Imperial defenders had to further hold out for the next four weeks.

The Imperial forces were forced to bombard the inter-lands and parts of cities just to slow down the Tyranids' inexorable advance. They were even forced to abandon a hive city when they could no longer hold it, but they managed to hold out and receive much-needed aid when their regiment and several other additional regiments from Cadia arrived. They drove back the Tyranids until they reached the Lost Hive city. However, there were millions of Tyranids all over the city and it would be too costly to retake.

Officially, there is no knowledge that the city had any survivors after the evacuation and retreat from the city. So the Imperial forces opted to bombard the hive city until there was nothing but a crater of rubble and dust. As for the Hive Fleet, Therone was completely destroyed, down to their smallest ships and troops. The hive fleet is believed to have been utterly destroyed, but was later found again decades later.