A breakaway band of Changelings who rejected the peace that ended the Dominion War and, led by Captain Vadic, infiltrated Starfleet in alliance with the Borg ahead of Frontier Day 2401.
The rogue Changelings are the main adversaries for much of Picard's third season. The faction has no formal name on screen. Its founders were prisoners of war experimented on at Daystrom Station under the secret Project Proteus, which made them able to mimic solid bodies well enough to pass blood tests.
Vadic escaped, shared the ability with others and broke with the Great Link. From her ship, the Shrike, she hunts Jack Crusher while her agents replace Starfleet officers and steal Picard's original body from Daystrom Station.
Dominion reveals her history, and she dies in Surrender. Vox and The Last Generation show that the group served the Borg Queen's plan to assimilate the fleet; the surviving infiltrators are afterwards exposed and arrested.