Forerunner production

A pyramid five times the height of the Great Pyramid of Giza sat kilometers from Kurt's vantage. Instead of stone blocks, however, the structure was composed of floating golden spheres that turned and glowed with Forerunner hieroglyphs etched upon their surfaces.

Every six seconds a sphere from the apex of the pyramid ascended in a shaft of silver light. As it rose, the light intensified so even with maximum polarization on his faceplate Kurt could not discern what occurred there. When the sphere emerged, three rods accompanied it, all parts spinning in null gravity, flexing, until the pieces settled into their deadly recognizable configuration a Sentinel of Onyx.

The new drone flew off into the clouds overhead which Kurt could only estimate were thousands of completed units.
(Halo:Ghosts of Onyx; Ch.35)