Underground

"They've remained behind in the Outer Rim, Wild Space, heck, even the Unknown Regions—continuing the fight where the regular military abandoned it."

- Anhri Claynes to Calen Solayr on the 'Underground'. The Triumvirate Subversive Intelligence Service, often simply referred to as the SIS or informally as the Underground, was an intelligence organization founded by the Galactic Federation Triumvirate for use during wartime in order to acquire and pass on damaging information about the enemy. With the invasion of Raixan Fauth and his armies, however, the SIS was partially reorganized into an underground force, stationed and working in the already-conquered Outer Rim and Unknown Regions, acting as a somewhat guerrilla force and launching operations that would frustrate Fauthian attempts to gain further control in the affected areas, or impede his invasion effort in the rest of the galaxy, as well as ferrying refugees from already-conquered worlds to the comparatively safer Mid Rim and Core Worlds. They worked heavily with the New Jedi Order and Jedi often took key roles in their missions and battles.

They were initially primarily staged on a hidden planet known informally as Hideaway, until the arrival of Calen Solayr to the planet, with Fauthian forces following not too far behind. Deciding to hold their ground, they engaged Fauth and his soldiers in their first formal battle of the war, ultimately managing to destroy the Dark Crown superweapon Fauth deployed and successfully damaging large portions of his fleet, forcing him to order a full retreat, marking the first real victory in their campaign. They continued to act against Fauthian forces, including harboring and protecting Jedi during the following Second Great Jedi Purge and searching desperately for Fauth's homeworld in the hopes that doing so would give them the key to defeating him.

Following the Fall of Coruscant, remaining Triumvirate forces joined together under the SIS banner as the Fauth State became the dominant galactic government.