Anodyne Spirit

History
The Forerunner Dreadnought, also known as the Key Ship, was built by the Forerunner and was part of their plan to activate the Halos. The ship itself has a design characteristic of all Forerunner constructs; it is very geometric and angular with no apparent rounded surfaces. It is estimated to be about 14 kilometers (8.694 Miles) tall, travels vertically like a rocket and can enter Slipspace. . At some point, the ship was found and used by the Covenant to power all of High Charity.

Storyline
The Dreadnought was found by the San 'Shyuum during their war with the Sangheili. Albeit only partially operable, it decimated the Sangheili warships in hit-and-run attacks, until the Sangheili began disassembling Forerunner artifacts and building their own versions. Then, the playing field significantly leveled, the two groups fought to a standstill, and reconciled. They wrote the Writ of Union, which formally began the Covenant, and decommissioned the Forerunner warship, removing all known weapons, and placing it within the (at the time) partially completed High Charity.

From the beginning, the Dreadnought is central to the Covenant religion. However, Covenant scientists and worshipers are split over the usage of the ship, as scientists wanted to further disassemble and study the warship to derive more technology and knowledge from it, whereas worshipers were stymied at the very suggestion of tampering with it. Compromises were apparently made, as Covenant technology is nowhere near the Forerunners level, but some pieces of technology have been derived, such as the Luminary, not to mention using the vessel as the primary power source of High Charity.

When the Flood attacked the city, SPARTAN-117 was on his way to stop the Prophet of Truth from escaping and at the end of Halo 2, he was able to board the ship as Truth uncoupled it from the city's power grid (presumably, it was the Prophet of Truth that was using it since High Charity was overrun by the Flood and he couldn't use that safely to get to Earth). It then left the system with the Halo and headed for Earth.

Soon after it arrived in orbit around Earth, Lord Terrence Hood was about to give the order to destroy it. He only stopped because he found out that the Master Chief was aboard.

The Dreadnought then landed in Africa, a continent on Earth.

The ship landed in the center on what would come to be known as the portal to the Ark. It is probable that the Forerunner Ship is an undestroyed Key Ship, which would activate the Ark portal. This is evidenced by its apparent activation of the Ark portal on Earth while it was landing in the middle, and Truth calling the Ark portal on Earth it's "true resting place" during gameplay dialogue. However, while its presence there during activation may be mere coincidence, the fact that it was used to directly channel Ark portal energy makes this unlikely. After the portal activation, the Dreadnought left with all of Truth's ships through the portal. At this point it is not seen again and it is unknown what happened to it. It is likely, however, it may have survived because of how resilient it was, the evidence being that immediately before the Ark Portal activation, the ship itself was attacked by Lord Hood's ships and took several direct MAC Cannon hits without sustaining any apparent damage. It probably had landed on the Ark as it did not take part in the battle above the installation.

Trivia
It is possible there is a Forerunner A.I. in the ship, the evidence being the presence Cortana found while trying to slow its launch sequence. The events of Halo: Contact Harvest confirm this, but indicate that the Dreadnought had an Oracle of it's own. This Oracle is mentionable because it is the cause of the Human-Covenant War, telling the Minister of Fortitude that humans were Forerunner.

According to the account by Forerunner AI Offensive Bias in the sixth Terminal, he controlled dreadnoughts exactly 37,654 tonnes each, in his battle with the Forerunner AI Mendicant Bias. This may be the same class of ship although it is likely it was a special class of Dreadnought designated the Key Ships.

"Dreadnought" may be just a classification for the ship and it might be the Sharquoi mentioned in Conversations from the Universe.