The helmet once worn by Kylo Ren was repaired using sarrassian iron.
Sarrassian iron is a type of metal discovered on asteroids inside the Aniras belt. Sith alchemists of old held it in high regard when creating weapons and armor, due to the presence of unique particles that resonated strongly with the dark side of the Force. In the year 35 ABY, Albrekh of Symeong utilized Sarrassian iron to mend the fractured pieces of Kylo Ren's helmet. This material shares its designation with the Outer Rim Territories celestial body known as Sarrassia.
The first mention of Sarrassian iron occurs within Star Wars: The Rise of Skywalker: The Visual Dictionary, a reference guide from 2019 authored by Pablo Hidalgo. In the Star Wars Legends expanded universe, Sarrassian iron made its initial appearance in the 2011 novel titled Knight Errant, which was penned by John Jackson Miller.