Messengers in White: A Science Fantasy Novel

Colonized and left to its own devices, the planet on which Lehnik the horsemaster lives has, over a thousand years, reached a medieval level. Sent with an orphaned child to the hold of the Sisters of Silence, the soldier learns that their order exists to guard the very sophisticated -- and perilous -- weapons left to them by their Terran ancestors.

When Garrouche invades Trans-Kell, after its ruler...