From Destinypedia, the Destiny wiki
The Oracle Engine is an arcane device located beneath the Observatory in the Dreaming City. It functions as a means of connecting different planes of reality, either for communication or transportation.
At some point after the construction of the Dreaming City, three Techeuns became possessed by an unknown group of entities. These beings used the Techeuns to address Mara Sov, who identified them as "the Ancients". During their conversation, the body of one of the Techeuns, Kelda Wadj, collapsed into a singularity. This singularity was later used to construct the Oracle Engine.
War for the Dreaming City
After the unsealing of the Dreaming City by Uldren Sov, Petra Venj and the Guardian used the Oracle Engine to communicate with the Queen. Later, the Guardian would use the Engine to visit the Queen in person.
List of appearances