Tanith First and Only Cross-Over. Equestria joined the Imperium of Man, and due to Imperial Law homeland of ponies need to provide Imperial Guard Regiment of their own. Colonel-Commissar Ibram Gaunt is sent to ensure everything is going well.
A lone Korpsman is sent to wipe out Equestria, but first he must dig a trench. And dig...and dig...and dig. Destroying Equestria is going to take a while.
The rebels planned things well. They've broken through and Celestia is the only one left between them and the throne. Now all they have to do is take it.
When a young pegasus with a gift for astronomy attempts to make a career out of his ability, he attracts the attention of several ponies, including the Princess of the Night herself...