The Wardens were a rogue Borg collective accidentally created on the planet Vulcan which, in 2377, almost succeeded in taking over the galaxy.
The Wardens were created by accident, by a group of Vulcans attempting to cure their master, N'Kal, of Bendii syndrome by using modified Borg nanoprobes. The Vulcans lost control of these nanoprobes, but rather than being assimilated, N'Kal was altered - the Borg's programmed desire to spread perfection was combined with N'Kal's thirst for logic. The modified nanoprobes quickly spread to the research team, making them drone-like followers of the Wardens' desire to spread logic.
The modified nanoprobes, which the Wardens named nanites, did not inflict the usual modifications associated with assimilation. The only difference between a warden drone and the person in their normal state is their mental condition: like a Borg drone, a warden become part of the collective, with N'Kal effectively acting as the Borg Queen. However, unlike a typical drone, Wardens could not be identified by technological augmentation, as none occurred.