Anodyne Spirit

History
The Forerunner Dreadnaught, 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, its 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. It is speculated that because the ship was able to power the entire city, it can travel faster then any other ship in existance at the time the Halo events take place.

Storyline
The ship itself has a prominent role in the overall story line and was High Charity's power source. 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 Dreadnaught 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 landed 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 Dreadnaught 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. However, it is likely it may have survived because of how resilent 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 witout sustaining any apparent damage. It presumably landed on the Ark as it did not take part in the battle over it, and was presumably destroyed when the Master Chief, the Arbiter, Sergeant Johnson and Cortana fired the Replacement Halo: destroying it, the Flood and the Ark.

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.

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.