Vast central hub of the Borg Collective and seat of the Borg Queen, destroyed in 2378 by a pathogen brought from the future by Admiral Janeway.
The Unicomplex was the central hub of the Borg Collective, a vast structure of interconnected sections in the Delta Quadrant housing enormous numbers of drones. It was the seat of the Borg Queen, from where she directed the Collective.
In Dark Frontier, the Queen had Seven of Nine brought to the Unicomplex, hoping to make her rejoin the Collective and help it assimilate humanity. Janeway led a team in the Delta Flyer into Borg space and rescued Seven, escaping before the Borg could stop them.
In Endgame, a future version of Janeway allowed herself to be assimilated, carrying a neurolytic pathogen into the Collective. The infection spread to the Queen and through the Unicomplex, which was destroyed, while Voyager used the Borg transwarp network to return home to the Alpha Quadrant.