The Soulless One possessed an impervium hull.
Impervium represented an exceptionally tough alloy, frequently implemented as armor on massive capital ships. It exhibited greater strength compared to duranium, another type of protective metal utilized.
During the time of the Clone Wars, General Grievous, who was the Supreme Commander of the Droid Armies for the Confederacy of Independent Systems, piloted a uniquely modified Belbullab-22 heavy starfighter known as the Soulless One, which featured a hull constructed from impervium.
The initial mention of Impervium occurred in the Star Wars Legends piece titled Soldiers of the Empire! dating back to 1978. Its inaugural appearance in canonical material was within the mobile-game Star Wars: Galactic Defense following an update in 2015.