Hands of the Emperor (The Horus Heresy Short Story)

There are no more loyal servants of the Master of Mankind than his own Custodian Guard... except, of course, for his appointed praetorians, the stalwart legionaries of the Imperial Fists. When Shield-Captain Enobar Stentonox is assigned to watch over the Palace, it is only a matter of hours before a full alert is sounded - secure airspace has been breached, by none other than their well-intentioned Space Marine allies. With neither side willing to accept responsibility for the mistake, tensions rise and a new battle begins in the skies over Terra.
Read it because
A giant floating city could crash into the Imperial Palace on Terra! This is a tense and unique tale, pitting loyalist against loyalist in a time where they should be sticking together. Can they stop bickering in time to stop the Palace's destruction? Read on to find out.