Crossover with the Battletanx series. It's unnecessary to know it beforehand. Everything pertinent is in the story. TL;DR Big tanks = Big booms
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The world has suffered from a mega virus that targeted and wiped out 99.99% of all females regardless of species. Only a few non-alicorns survived the catastrophic event and now exist as the Queen Lords of their people, surrounded on all sides by loyal males that have come to worship them in these dark times. Shining Armor and Cadance have been on the run with the remnants of the Crystal Guard, looking for a safe home in this ravaged wasteland to raise their soon-to-be child.

With the military weaponry having been raided, replicated, and reintegrated into society, the world is a battlefield of death and destruction crushed beneath the treads of the battletanx. Only perseverance will see the couple through these apocalyptic times and reach the promised lands when the sun itself comes to burn them.