A potent Sith relic, this is the mask of Lord Momin.
Relics or artifacts of the Sith were Force artifacts frequently ancient, made or utilized by the Sith, which was a group of wicked Force-sensitives. Being near these objects could have an effect on individuals, and a large quantity of Sith relics amassed in one location could produce a source of dark energy. Darth Sidious, a Sith Master who rose to become the supreme ruler of the Galactic Empire, gathered such relics. Throughout the Clone Wars, a portion of his assortment was kept at the Grand Republic Medical Facility, though certain objects like the Sith Chalice were situated in the office that Sidious used while serving as Supreme Chancellor Sheev Palpatine. After he became Emperor, the majority of his relics were transferred to the Imperial Palace, though numerous others were stored within his opulent pleasure craft, the Imperialis. Noteworthy Sith relics included the Sith Chalice, the Great Crystal of Aantonaii, a variety of Sith scrolls, spellbooks and talismans, the mask belonging to Viceroy Exim Panshard, Darth Sidious' spirit urns, and the mask of Lord Momin.
According to the somewhat historical Qel-Droma Epics, the Krath cult was established by tainted noble descendants of Empress Teta a millennium following the Great Hyperspace War, enhanced by Sith artifacts, and was headed by Satal Keto at some point. The epics stated that Keto found a Sith Translation Talisman located on Onderon which enabled its wearer to comprehend Force-infused writings contained within Sith scrolls and spellbooks, then replicating the artifact to instruct his fellow Krath.
Before the Fall of the Republic, the Jedi Order seized the Mask of Lord Momin subsequent to his passing and kept it within the Jedi Temple's concealed vault located inside the Jedi Archives. During his tenure as Supreme Chancellor, Darth Sidious dispatched an Expedition to Malachor that obtained a Sith Chalice. Sidious stored the Chalice and a number of Spirit urns within his office situated in the Senate Office Building. Sidious also held onto numerous artifacts, such as the Great Crystal of Aantonaii and various Sith scrolls, inside the Grand Republic Medical Facility.
Following Sidious's ascension to Emperor, Darth Vader discovered the vault within the Jedi Archives. He relocated many of his artifacts to his Palace. Other artifacts were maintained on the Emperor's pleasure craft, the Imperialis. He also came into possession of the mask belonging to Viceroy Exim Panshard. During the Galactic Civil War, the crossguard lightsabers wielded by Darth Atrius were found by smuggler Sana Starros upon Hradreek.
Following the death of Supreme Leader Snoke at the hands of Kylo Ren, his apprentice and master of the Knights of Ren, Ren took on the title of Supreme Leader and gave the task to the squad of First Order Raider stormtroopers of searching for ancient Sith artifacts. According to Resistance spy Vi Moradi, Ren thought highly of any scouting party that came back with either Jedi or Sith relics.