The Separatists are Upyri that were formerly part of the Insurrectionist forces during the Division War. They did not agree with the surrender of Ker and most of the other Insurrectionists to the Peacekeeping forces of Hemera and Nyx, so they decided to continue their fight. Most were driven from Upyri space, out into the galaxy.

Most of these rogue Upyri forces have been captured or killed, but there are still quite a few that still oppose the current Federation government and those that ally or trade with it. They have become a bit of a nuisance for trade ships, as well as foreign factions landing on seemingly uninhabited worlds. The previous government of Sol encountered a problem with these rebel Upyri, which lead to first contact with the rest of the galaxy for Humans.
Upyri Federation of Planets
Military ForcesONICEEThe Honor GuardFBLE

WeaponryMechs & Battlesuits
Territories: NyxHemeraKerMoirai
Combat Vessels: Spatha-ClassDominion-ClassCaliburnus-Class (RamlethalElphelt) • Schiavona-Class (2410s) • Xiphos-ClassGladius-ClassJavelin-Class (2410s) • Sica-ClassFerrum-ClassDardus-Class
Non-Combat Vessels: Sagitta-ClassSalvator-Class
Post-Interstellar History: The Division War (The Graveyard of MoiraiThe Separatists) • Expansion into the Southern QuadrantsUpyri in the Renegade Quadrant
Pre-Interstellar History: The Blood WarsAncient UpyriThe Colonization of Hemera
Vehha StryeTirell MetaisMarus ViraKetri BiramJiral NeaarsVedok PietschBastian Veiel
Former Members: Anora Cortus

Crimson Incarnate
Upyri Federation Peacekeeping Forces
Factions: The Empire of Ker (Ker RisingKer Supremacy Movement)
Lore: The Second Division WarHyperwarp TechnologyBeyond the Edge