A Shield World is a Forerunner installation, designed to be a shelter from the activation of the Halo Array. The main component of a Shield World is a complete and thorough Micro Dyson Sphere, located within a Slipspace bubble of compressed space-time. The only way in is a small portal that constitutes the only real-space component of a Shield World. Access to Shield Worlds was only possible upon the imminent activation of the Halo Array.I found it. :D
Did you even read Ghosts of Onyx before you asked this ?