The Iron Choir is a group of people who worship the unmoving body of Peace and "dead metal" -- metal that has been disconnected from the Mesh. They live in Seabed City, an abandoned dome, where they eke out a living far from the other populated domed cities of Counterweight.
Their symbol is an upside-down peace symbol "almost like a Y with an extra line going through it at the top".[1]
Addax Dawn apparently retains some contact with the Choir (as he mentions speaking to Sister Rust in a journal entry).[1]
This section contains spoilers for COUNTER/Weight 43: A Splintered Branch, A Ringing Bell Pt. 3.
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By the finale of COUNTER/Weight, the Iron Choir is loyal to Aria and her Righteous Vanguard.[2] Under Sister Rust's direction, they aid in the fight against Rigour alongside Tea Kenridge in an army of dead metal mechs. This is "seen as a sort of betrayal" of their principles, but they consider Rigour a greater enemy than live metal. In the final moments of the battle, Peace is overtaken by Rigour, and Sister Rust willingly activates the golden orb to stop the Divine, causing her death.[3]