From Halopedia, the Halo wiki
REQ-814-D is a location on the Forerunner shield world Requiem. The structure is identical to site REQ-814-B, colloquially known as the "Fortress".
In early 2558 this location was held by Prometheans, and it was north of the UNSC's operation of influence. Fireteam Majestic was sent to the structure to deactivate a Forerunner tower under the supervision of Commander Sarah Palmer on February 9th, 2558 as part of Operation: CHATTERBOX. After taking down Forerunner communications jammers, Majestic cleared the area of Prometheans and deactivated the Forerunner structures and were extracted.