Composer

"In the Forerunners quest for transcendence, The Composer had been intended to bridge the organic and digital realms: it would have made us immortal. But its results soured."

- The Librarian

The Composer is a piece of Forerunner technology that was created by the Forerunners in order to gain a foothold against the Flood in the Forerunner-Flood War. It was capable of bridging organic life and digital technology together, and was used on ancient humans, resulting in the creation of digitalized organic beings called the Prometheans. Created when The Didact refused to give up the Mantle, the Composer was originally intended to allow the Forerunners to become immortal. The Didact pursues this piece of technology in Halo 4.

Pre-2557
It is unknown exactly why the Composer was made, but one prominent use of this device was to convert Chakas into 343 Guilty Spark to preserve his mind.

In Halo 4, it is revealed that the Composer was used by the Didact to transform humans into Prometheans, a process he refers to as "Composing". However, the device was flawed; the stored personalities fragmented and attempts to restore them to physical form failed. The Didact believed that the Flood could be defeated this way, since they could only infect living tissue (the Prometheans being both digital and inorganic). However, the lack of moral values in the resulting Prometheans caused the Forerunners to abandon the project, most likely due to the fact that as humans were seen as being the next race to uphold the Mantle, they had to have moral values. The Didact ignored this however and continued using it. When the Didact used the Composer on a population of humans settled on one of the halos, the Composer was confiscated from him and hidden on Installation 03 by the Librarian, halting the process completely for the next hundred millenia.

Post War


The UNSC discovered the Composer on Installation 03 prior to 2557 and had the Infinity move it to the adjacent Ivanoff Research Station for study, a process which took 3 months according to Sandra Tillson. While researching the Composer, several members of the team were affected by it and reduced to ash, prompting the team to be more careful with working on it. Sensor data found after this incident gave coordinates that lead to Requiem.

In 2557, the Didact escaped from Requiem and attacked Ivanoff Station with his Prometheans and the Storm Covenant. He successfully captured the Composer and fired it at Ivanoff Station, instantly killing all on board except for the Master Chief, who was immune to its effects due to the Librarian's genetic manipulation. The Didact brought the Composer to Earth to destroy humanity and was even able to fire it at Earth. The city of New Phoenix was hit, and every human in it was instantly "composed". Master Chief was able to board Didact's flagship, place a grenade on his chest (as multiple versions of Cortana were binding him down) then destroy the Composer and the flagship with a Havok nuclear bomb, ending the threat for good.