Anodyne Spirit

Forerunner Dreadnought The Forerunner Dreadnought, also known as the Key Ship, was built by the Forerunners and was part of their plan to activate the Halo Rings.

Background
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 (8.699 Miles) tall, travels vertically like a rocket and can enter Slipspace. .

Sangheili-San 'Shyuum War
Early in their history the Dreadnought was found and worshipped by the San 'Shyuum. These early worshipers were split over the usage of the ship; the Stoics refused to enter the vessel while the radical Reformists wished to explore it. The Reformers then barricaded themselves within the Dreadnought and fled the San 'Shyuum homeworld, taking a chunk of the planet with them. It was used in the later Sangheili-San 'Shyuum War where it was able to decimate the Sangheili warships in devastating hit-and-run attacks(although it was only partially operational), until the Sangheili began learning from Forerunner artifacts to improve their own ships. This then brought the war to a virtual standstill and a peace was finally established.

Writ of Union
The two races then penned the Writ of Union, which formally began the Covenant, and decommissioned the Forerunner warship, removing all known weapons, and placing it within (at the time) the partially completed High Charity. It was then used to power the city.

The Great War
At the beginning of the Ninth Age of Reclamation the ship's A.I. was given data that concerned the location of a human world, Harvest. This in turn caused it to start a launch sequence to travel to the planet. It was only stopped when Lekgolo worms were able to short the control systems and then soon after the A.I. itself was isolated from the rest of the ship. Had the ship launched, it possibly could have killed every living being in the city, as the atmosphere would have vented into space.

The Flood Infestation
When the Flood attacked High Charity, 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.

Second Battle of 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.

Battle of Installation 00
The ship landed in the center on what would come to be known as the portal to the Ark. Though it is never explicitly stated, the Forerunner Ship is most likely 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 its "true resting place" during gameplay dialogue. 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, but in in-game dialougue truth gibbers about "My dreadnought cannot rise...". 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 and dozens, perhaps even hundreds of bombs dropped by Longsword fighters without sustaining any apparent damage. It probably landed on the Ark as it did not take part in the battle above Installation 00.

Trivia

 * The ship itself has given the Covenant much knowledge, an example being how to construct a Luminary.
 * 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 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.
 * The Dreadnought is thought to be the fastest ship in existence at the time the Halo incidents take place as its able to power all of High Charity even when not at full generating capacity.
 * It is mentioned that the vessel removed a "chunk" of the Prophet homeworld when the Reformers fled . This may allude to an explanation of the planetary dome of High Charity.