Precursor tech being used at minimum effectiveness by the Covenant Remnant. It's why blasting two plasma batteries "disables" it; the Remnant isn't using it effectively. In the next few episodes you manage to revive the Didact in a Control Room giving him access to the entire Forerunner translocation array and he teleports Miller, Palmer, Roland and Crimson to the Greater Ark before he gets a mutation from the Librarian bringing him back to his good Ur-Didact state. Beating all 10 Episodes on Legendary, watching all the terminals and CG episodes unlocks Episode 11 - Epilogue, taking place with Crimson on the Greater Ark, which is essentially being assimilated by Precursor technology. Crimson gets "integrated" with it and uses two new weapons - a Bore and a Mortar - in order to take down a fully-functioning, mobile Harvester in the clearing in front of the Greater Citadel in hopes of making it to a comm relay, but ultimately were misguided by the Last Precursor. The Precursor kills Crimson, Roland and Miller, but teleports Palmer back to Infinity essentially telling everyone the Precursors have returned and goes mad from the revelation, killing herself punctuated with a fade to black. This kicks off the events for Halo 5.