Darth Cyrus' Sith Remnant was the government that formed of the remnant of Darth Cyrus' Sith Empire in 139 ABY by Darth Cyrus. It became founder and crucial member of Dark Reach in the years following.
Founded in 139 ABY, the government was indirect successor of Darth Cyrus' older Sith empire. Cyrus named his new empire the Sith Remnant, being located in Korriban and it's neighbors, as a result being composed of many Sith. It would build up for the following two years, and found Dark Reach. Cyrus corrupted many independent governments, most prominently the Yevethan Protectorate, into following him. The Yevetha joined Dark Reach and prepared to engage the current Galactic Alliance Remnant.
Cyrus and his new found allies prepared to fight the Alliance Remnant, until a turn of events caused the Yevetha to kill the current Alliance Remnant leader, forcing it into reorganization due to a newly arisen threat. Many more independent governments joined Cyrus, while most forcefully, some willingly allied themselves with Dark Reach, like the Predecessors.
The Sith Remnant's first battle was the battle of Kashyyyk, which ignited the Infinity War. One of the Dark Reach member states, the Aradan Sectorate, was soon attacked by the newly formed and successor for the Galactic Alliance Remnant, the Federation of Free Planets. Cyrus' Sith Remnant was among the few governments that moved to aid the Sectorate. Even though the Sith Remnant indefinitely lost, their casualities were held to a minimum, falsely making the Federation think they've won, while Cyrus was instead preparing to launch an offensive against the Federation capital.