Sombra conscripted much of the population into his new army, while the rest would work to forge weapons and gather supplies to feed the war effort. His experience and understanding of tribal military tactics allowed him to revise strategies capable of using the freezing conditions and mountains heights to their advantage, resulting in many incursions North ending in disaster for the invaders. Army after army came North, driven by the promise of riches, especially the crown jewel of the empire, the Crystal Heart, as well as by a desire for a place in the history books as the conquerors who finally broke the crystal empire for good.
Prisoners of war were put to work like everypony else, and any resistance was met with death. In time, the once-peaceful Crystal Empire was tranformed into an efficient killing machine, led by Sombra's magic. The attempts at invasion became less and less frequent, and so the war became an uneasy peace.
But Sombra was not looking for a truce; he meant to press their advantage while the Equestrians were weakened, losing their taste for war and fawning over their new filly-princesses. He marched on the northernmost settlements and sacked them, dragging off their civilians to slavery. After that, he prodded deeper, hitting forts, farms, towns, and the odd fledgling city, as he made his way toward Canterlot. Soon, the new country was up in arms again, with multiple armies heading to halt his advance. They fought, thousands died, but in the end Sombra was forced to return to the North, defeated.
The crystal ponies asked him to cease the war and attempt a peace, as the Equestrians did not even try to follow them back home. But Sombra shut himself away from his subjects in the crystal tower, not speaking or eating for days at a time. It was in this time that he truly made the transformation to Sombra, the slave king, as he'd been experimenting with his powers to gain some kind of edge on the Equestrians.