Its tetrahedral shape and massive size reflects the Forerunner theme, it was also designed with its unusual shape to be stable on ground. The Forerunner Dreadnought is weighed by thousands and contains a lot of facilities and thousands of rooms in its interior. The ship was used by the Covenant to power the High Charity, using only a slight amount of energy. The ship's origins ages about 100,000 years ago, when the Forerunners used to be existing.
This Keyship was used massively in the Forerunners-Flood wars, it was essential to put back the Flood. Most of it were destroyed to prevent it from being held by the Flood, few survived. Thousand of years later, it was sent to earth, infected by the flood. The Master Chief had to confront and deal with it.