Composer

The Composer is possibly a Forerunner machine that (presumably) can convert a sentient flesh and blood being into a Monitor or ancilla. It can also be assumed that the Composer was responsible for putting Chakas's mind into a Monitor (later called 343 Guilty spark) after the Didact and the Humans saved Installation 07

In Halo 4, it is revealed that the Composer was used by the Didact to transform living beings into machines, a process he refers to as "Composing". Didact believed the Flood could be defeated this way, since they could only infect living beings. Didact used the Composer to transform his Prometheans into machines, however the Flood proved too numerous for his Prometheans alone. So, Didact began turning humans into Prometheans using the Composer. When the Librarian discovered his plan, she imprisoned him on Requiem and hid the Composer on Installation 03.

The UNSC discovered The Composer on Installation 03 prior to 2557 and moved it to Ivanoff Research Station for study. In 2557, the Didact escaped from Requiem and attacked Ivanoff Station with his Prometheans and Storm Covenant. He took the Composer and fired it at Ivanoff Station, instantly killing all on board, except for the Master Chief, who was immune to it's effects due to the Librarian's genetic manipulation. 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 killed. Master Chief was able to board Didact's ship and kill him, then destroy the Composer and the ship with a Havok nuclear bomb.